I'm currently doing 6x6 on my gridlock, sgould i make the bigger?
Over the years I have made every size possible. As long as you add good bus and subway and other transportation they all can work well. Its nice to break it up with variable sizes just like in RL. The larger sizes like 9x9 allow you to put plaza's and parks in the middle and even hospitals and schools. Variety is the spice of life.But yes 6x6 is a good standard.
Remember that the roads normally have only 3 tile "access" in SC4 so making roads wider than about 6x6 is kind of counterproductive unless you plan on putting "none traveling" stuff inside the middle.. Some ordinances extend it 1 tile.. I think its the "commuter shuttle" or something like that.. but still the comment about accessability applies.. And as H5T said, any size that floats your boat is fine.. ;)
Of course, they don't have to be square, and that 3 tiles would be satisfied as long as the one dimension is 6 or smaller. I have quite often built 6x8 or 6x10 even, and it works fine (though be prepared for traffic problems if you don't have a lot of mass transit).
I can put a 6 by 6 block, somethimes even 7 by 6, they work and I see by the residential blocks, I get flats that gives 500 people:D
I usually do 8x4 then 2x1 houses for terraces and rowhomes.