My childhood Playskool blocks (about 1970) are multiples of 35mm and I can't recommend them enough. The blocks are simply modular (1:2:4) and stable to great height.
(from left, by rows) (all dimension in millimeters)
* Half Column: 70 x 35 x 35H
* Square: 70 x 70 x 35H
* Triangle, short: (half of Square)
* Column: 140 x 35 x 35H
* Rectangle: 140 x 70 x 35H
* Triangle, long: (half of Rectangle)
* Arch: (same as Rectangle with Crescent removed)
* Crescent: 70 radius x 35H
The tolerances are close, four 35mm objects collected are the same length as a 140mm piece. (Necessary if you are trying to teach concepts of multiples or asymmetry.) The corners are probably rounded <1/16", and the opposing surfaces are parallel and smooth. (Also necessary if they are intended to stack to any challenging height.)
I think these were one of the more significant factors in my becoming an architect. My brother, myself, and neighborhood friends used them for a long time to build cities for Matchbox cars, and survived numerous chewings by German Shepherds. My parents used them to support various painting projects after that, and about five years ago I sanded them all down for my own kids to play with.
