If you check pedstats there is a line that says "Ped can be knocked down (0 = no, 1+ = yes)".
STAT_PLAYER none 20 100 12 200 0 0 0 0 50 95 0 0 0 0 0 60 Generic 0 7 100 1 0 0 20 100 20 10 10 10 20 100 80 100 50 70 50 90 0 0 unarmed None None None 0 unarmed -1 1 init unarmed -1 1 init unarmed -1 1 init unarmed -1 1 init 0 1 15
Change the second value from right to left, to 0 (next to 15).
The size thing in ide.img (small, medium, large, huge, fat) only affects the way a ped is bullied and the way they fall to the ground only if their stat allows them. For example Earnest is "medium" but since his value in pedstats is 0, he doesn't fall at all. Damon and Casey are both "huge" but Casey value is 1 so he falls anyway despite his size.