NB: I believe this topic has been discussed much better many times elsewhere on the net and in print. I am writing simply to clarify my own thoughts.
One of the perennial questions in urban design for those who get caught up in the details is the question of city block size. Since the organic texture of older cities such as Rome, Paris, or London isn't easy to prescribe, people instead focus their interest on the best layout of a planned city. And the size of an individual block often becomes a major decision. The answer is, of course, there is no perfect size, as there are trade-offs in any complex decision. With respect to block size, the primary trade-off is between walkability and taxability. The later really isn't a word, so a better term would be infrastructure efficiency.
The most basic infrastructure element of any community is the transportation corridor between the various properties that make up a community, whether they are residences, retailers, offices, schools, hospitals, warehouses, factories, or other less common establishments. In developed countries, most of the transportation corridors are dedicated to motor vehicle traffic. In American practice, the corridors are called streets or roads, depending on the context. Streets in urban areas usually consist of space for motor vehicles to circulate, flanked by space for pedestrians. They usually include sections for parking and for vegetation, and sometimes include dedicated lanes for cycling. The entire publicly-owned area between the other properties is referred to as the right-of-way. Roads in rural regions are generally built solely to accommodate vehicles. Suburban regions see a mix of street elements and can be called either roads or streets. Fancier names such as avenue and boulevard are usually applied to major arterials in cities, and highways to major inter-city roads.
Any transportation corridor that is more advanced than a dirt path must be paid for somehow. Usually, that is accomplished through taxes. In America, property developers often build or substantially upgrade the local streets or roads that pass in front of most residences, and then property taxes pay for ongoing maintenance. Excise and income taxes pay for new regional and national roads, and for their ongoing maintenance. The exact division of funding is determined by various agreements, the details of which aren't all that important here.
The efficiency element comes from the ratio of private, taxable space to public, tax-supported space. For instance, a 200' by 200' block (measured from the centerline of the street) with a 100' right-of-way (ROW) on each side leaves only a 100' by 100' plot of land which can be taxed. That is only 25% (100,000 square feet) of the entire block (400,000sf). Such a configuration would result in a lot of infrastructure for each property to support, increasing the taxes on each property. A more realistic ROW of 50' surrounding the same block gives a ratio of about 56%. A 300' square block bounded by a 60' ROW yields 64%, and a 600' square block with a 80' ROW yields 75%.
The other side of the trade-off is walkability. For the purposes of this discussion I am going to leave aside the debate over the value of walkability itself. But assuming it is a desirable quality doesn't do much to define it, of course. What makes an area walkable is a topic covered in a tremendous amount of writing by architects and urban planners. The elements that are often cited as contributing to the quality walkability include: the width of the roadway (if present), sidewalk (if separate), and bikeway (if present); the presence and specifics of vegetation; the presence and specifics of street furniture and other physical elements; the number of housing units, quantity of office space, and presence or absence of retail establishments; the size and aesthetic qualities of the surrounding buildings; and the size of a block.
Basically, big blocks are a chore to walk along and between. Explaining the unpleasantness of between blocks is easy - fewer streets means each street has to handle more traffic, increasing its width and the danger to pedestrians who cross it. The unpleasantness of along comes from the traffic of adjacent streets, and the fewer number of paths to a point. Cars - even electric cars - are noisy, and the more there are and the faster they go, the less pleasant it is for pedestrians nearby. If the cars are directly adjacent to the sidewalk, that can make pedestrians feel unsafe. Fewer streets also reduces the number of buildings, establishments, and other points of interest that a pedestrian experiences on a trip, making it less interesting. And finally, big blocks reduce the number of paths between two points, leaving a pedestrian fewer opportunities to shape a journey to pass things that are pleasing, or to combine different elements in a new way.
To get back answering the question in the title of the post, one way would be to look at existing examples. For instance, Manhattan is famously gridded, with the pattern established at 14th St. dominating the plan all the way up to the northern end. Blocks there are mostly 264' in depth, but vary in length between 500' and 900'. Many people have commented that the latter number is too large. However, Manhattan has a much more linear shape than most urban centers, which are not so tightly bounded by water, and the long sides of the blocks are perpendicular to the primary direction of travel. Chicago has a number of grid sizes, including 400' by 400' in The Loop, and 330' by 660' in the Little Village neighborhood. The planning of Phoenix is mostly dominated by car-oriented superblocks of 5280' square, and though they are subdivided finely close to downtown, very little is walkable. Barcelona has a grid size of approximately 440' by 440', and gridded portions of Madrid are similar. In Stockholm, blocks vary considerably, but many are about 330' by 560'. Turin, which has a core originally laid out by the Romans, also has a range of block sizes, with some as small as 180' by 280'. Grids in the Buenos Aires metropolis have several different orientations, but most blocks are about 440' by 440'.
Since that way provides no clear answer, only a range of values that isn't surprising, I will continue my exploration further in subsequent posts.
