There are several action cards that allow a PC to knock somebody prone i.e. Shield Slam or Corps-a-Corps. How would you handle a situation when the PC tries to use one of these on a bigger and stronger opponent? Throwing a few black/purple dice is OK, but I have a feeling that a 3rd rank character will easily pass the test, though.

 I might say one extra <P> per pair of legs or balance limbs (wings can be good for balancing), plus calculate the difference in weight and award [W] for each +100 lbs of the attacker or [B] for each +100 lbs of the defender.

 I just would not allow it. Probably I would put a size limit, like a PC cannot trip anything bigger than a blackorc or may be a troll.

The same for inmobilising shot.

 While I agree that being able to knock a dragon (or any other huge creature) prone is a bit silly I think you have to widen the interpretation a bit. Sure, the dragon will not be knocked over, but if the PC is skilled enough to actually succeed in making the action (dragons have quite a lot of defense), he/she should benefit in some way. Being knocked prone is only going to cause a character to have to spend a manouevre to stand up. Perhaps the shield bash momentarily unbalances the dragon slightly so that it has to spend a manouevre to right itself.

I mean, if you send the PCs versus a dragon you can't really start houseruling away their hard won abilities, that seems quite unfair to me. A dragon will be an extremely dangerous opponent even without that. If you really think it's a major problem I'd house rule that actions that knock opponents prone counts as opposed vs Athletics(S or T), giving at least 3 challenge dice for any PC that tries it.

I'm with Gruntl, the "mechanical effect" of the card can be kept while tweaking what it looks like in play.

If you feel the action seems too easy in the situation, then throw some misfortune dice on the action when used against a quadruped or similar "inherently more stable" creature etc.

While I am very lenient in allowing ym PCs some freedom, I must agree with Yepesnopes. The size difference would realistically make the attempt ineffective at rendering the target actually prone.

If you want to give them a chance (in the spirit of cinematic heroics), I would alter the listed test to be a contested Strength test, giving two extra challenge dice for the dragon being a quadraped.