What is the ideal "block" size in SimCity 5?

To answer this I'm going to need to establish some criteria for "ideal". I'm going to assume that most people are interested in the most efficient use of a given space for high density buildings allowing you to stuff as much population/commerce/jobs into your city as possible.

Luckily the game itself provides an excellent tool to help you with this. Turning on the "gridlines" feature when plopping a road tile will display gridlines for the last two road tiles you mouse over. The gridline displayed for a given road depends on what road you moused over and what road type you currently have selected.

Mouse over avenue with road selected: The first gridline will provide enough room for two high density buildings back to back making for maximum use of the space given the two road types you want to use.

Mouse over avenue with avenue selected: Same as above

Mouse over road with avenue selected: Same as above

Mouse over road with road selected: The first gridline will provide just enough space for a single high density building to exist between the roads. The second gridline will provide enough space for two high density buildings back to back and a gap inbetween that could fit a road (or is potentially useful for narrow parks).

Using the dirt road tool, here is how much space to leave between roads in simoleans. Numbers in parentheses were tried and found to be not enough.

§239 One HD Building between 2 Streets (238)
§253 Guideline - Street Hover + Street Tool 
§263 One HD Building between an Avenue and a Street (260) 
§287 One HD Building between 2 Avenues (286) 
§378 Guideline - Avenue Hover + Street Tool 
§437 Two HD Buildings between 2 Streets (436)
§461 Two HD Buildings between an Avenue and a Street (454) 
§485 Two HD Buildings between 2 Avenues
§505 Guideline - Street Hover + Street Tool (2nd line)
§505 Guideline - Avenue Hover + Avenue tool