Which is least likely to crack due to high traffic, large or small tile?
Smaller tiles are less likely to crack on the tile itself, the joints will give first.  But larger tiles are stronger in general.

That said, you should work on increasing the strength of your floor before tiling if you are concerned about cracking.  If you can access the joists, sister any weak places between two load bearing points.  On the floor itself, install a cement board with lots of screws along the joists.  Stagger any joints from the subfloor below.  And make sure the entire surface is perfectly flat (level is good, flat is required).