I don't know what materials you are using -- lightweight resin?
Figure things this way. The average single 1/35 scale model is about 9" (227mm) long and 4" wide (102mm) so you need a base about 8 x 12" (203 x 304mm) to provide surrounding space. Also the center of focus needs to be skewed slightly so the model sits at a bit of an angle on the base.
As for the base itself:
-- cobblestone streets with a curb to one side are good, perhaps with a streetlight or simple roadsign
-- asphalt road section with a small ditch on one side
-- dirt road with flat surfaces (planted tank track marks are not good unless you have them match the subject, as well as the fact that the tank or vehicle will then tend to "hover" over the road.
-- concrete pads (e.g. modern motor pools)
The same things could be used for 1/72-1/76 but about 1/4 the size.
Also you may have to think about 1/48 as it is making a comeback -- about 5 x 7 or 6 x 9 would be right.