The story goes that an English tourist, on holiday from London, unaware that he stood in Times Square, asked a policeman for directions to Times Square.  The policeman, smirking at the stupid Limey, instructed him to take the next left, then another left, then a third left before taking the next left after that.  The clueless Cockney, used to a city where four lefts will take you a good mile from where you started, promptly did so, only to discover himself back in the same place, with the policeman laughing at him.

A CityBlock, much like a CountryMile, is whatever you want it to be.
Y'all have blocks in cities other than "America", now don't you?

''No, we don't, but you probably knew that. Did I miss the IronyWarning?''

Actually, many do. Several European capitals have a traditional center, with streets haphazardly laid where feet have trod, surrounded by a 19th century "ring" buildup of more-or-less rectilinear streets. Many cities in South America are laid out in a "North American" style gridiron as well.

Scotland has plenty of gridded layouts.
In Philadelphia, I will walk a block in about a minute (and drive it in two).
It varies wildly though.  Using the term 'block' is meaningless unless you have context.  

Typical Italian cities have no regular blocks, or so few that they look strange; the blockiest layout that can be expected is a grid of reasonably (but not equally) spaced parallel and perpendicular streets.
Ancient Romans actually built cities with straight and exactly perpendicular streets, and probably with regular blocks (expanding the standard military camp layout); but after their age there have been many centuries of haphazard rebuilding, sometimes with deliberately messy and complex layouts and narrow streets.
Modern and contemporary city developments in Italy are very limited in scope and normally of irregular shape (e.g. an abandoned factory), so the only common regular patterns are sets of twin buildings in the same lot and long, straight roads between lots. 
I was surprised when I read that there were places where city blocks could be used as a unit of distance. -- LorenzoGatti

A major part of Amsterdam is circular, or rather semi-circular. A lot of old city centers are whatever shape they happened to be, while most newer parts of towns (so called Vinex areas) have maze-like structures.
While a grid layout will make giving directions a ''lot'' easier, it also makes life less interesting. ''Can someone please generalize this into an observation about the USA?'' -- AalbertTorsius.

Until the 19th century, cities were generally fortified, and the street were laid out in a way that made them more defensible. Long, straight streets are generally to be avoided; the invaders can see what is coming and clear the way with cannons. Preferably, you have narrow roads with lots of corners; every corner can be used as a line of defense. Having narrow streets with high houses and even the occasional gate means that the invading enemy is basically walking through a kind of shooting gallery when entering the city. The defenders would know all the shortcuts, which alleys were dead ends, and all the other details which would help them to outwit invaders.

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From AmericanCulturalAssumption:

''16 American City Blocks would make one CountryMile. If they were in the country [meaning "countryside" or "rural areas"]. -- PhlIp'' ''''' -- How did you come to this conclusion?'''''

(Most?) American cities are laid out with 1/16 mile by 1/8 mile grids. (Metric equivalents: 100 meters by 200 meters.) Major streets are usually at 1/4, 1/2, or 1 mile intervals. (Metric equivalents: 400 meters, 800 meters, or 1.6 km)

Some exceptions: Midtown Manhattan (in New York City) has a rough 1/20 mile by 1/10 mile grid, with some avenues being twice that length at 1/5 mile.

Newer neighborhoods (often called "suburbs") usually have grids of major streets, but the minor streets are often mazes instead of grids.

Streets in Salt Lake City are 7 to the mile, in both dimensions. An oddball number, but the consistency (plus the use of numbers for all addresses) makes calculating distances straightforward.

''A city block is the distance between consecutive streets, running east-west, or avenues, running north-south. The Manhattan grid has about 20 streets per mile but only a few avenues per mile making it convenient to describe "short blocks" or "long blocks" (for blocks facing avenues or streets respectively). Portland, Oregon was laid out with most streets and avenues in a 200 foot grid, making more corner lots so that developers received more profit as corner lots command a higher price.''

How exactly do you come to the conclusion that E-W is a 'street' and N-S is an 'avenue'?  Last time I checked, 'street' is a road built up on either or both sides, and 'avenue' is a tree-lined road. [''That's just the way numbered roads are laid out in Manhattan and some other places. It would surely be less confusing to use sets of numbers that don't conflict, particularly for visitors from places without that convention who don't suspect the vital significance in the "avenue" or "street" after the number, but that's how they named 'em.'']

''There is no definition of how big it is. Each city block is just as big as it is. They aren't even all the same shape.''

''A city block would typically be 1/16 to 1/8 of a mile, a football field is 300 feet, Labor Day is the first Monday after the first Sunday of September and is viewed as the end of summer vacation.''

In many large eastern cities, a CityBlock is a standard 1/20 of a mile. That is, there is that much space between the centerlines of the streets in grid-platted parts of the city.

''Here in the SouthWest, though, we have grids of "major" streets spaced about a mile apart, sometimes more, with minor streets running through them.  Measurement in blocks is meaningless to me.''.

In Idaho, as in many parts of the West, roads in farming country (at least the flat spaces) are often spaced a mile apart, and run directly N-S or E-W.  The big exception is when roads follow geography like a mountain or river.  A good rule of thumb for city blocks is 1/8 mile.  The street address numbers almost always increment 1000 for every mile and 100 for every block.  (If you're adding these up, remember, I said 1/8 is a rule of thumb!)  The numbers increase from the center of the city, going out.

Cultural assumptions, again. Outside the USA, '''and in many parts of the USA,''' a city block is not necessarily rectangular, or even close. Or even anything anyone would consider as denoting a given distance.

I was under the distinct impression that the actual size didn't matter. It was merely a tool for giving informal directions ('Walk three blocks that way, then take a left and walk for three more blocks' means 'Walk by three main streets then go left and walk by three more main streets') or a general impression of size -something big. It is all very informal and I suppose every can see that from the discussion so far.