Any opponents hit by your shield bash are also hit with a free bull rush attack, substituting your attack roll for the combat maneuver check (see Combat). This bull rush does not provoke an attack of opportunity. Opponents who cannot move back due to a wall or other surface are knocked prone after moving the maximum possible distance. You may choose to move with your target if you are able to take a 5-foot step or to spend an action to move this turn.
So, any opponant hit by your shield bash is hit by a free bullrush attack. Does that mean that it bypasses the standard size category rules?
If that bypasses the standard rules, how does that combine with a Siegebreaker's Breaker Momentum
At 2nd level, when a siegebreaker successfully bull rushes a foe, he can attempt an overrun combat maneuver check against that foe as a free action.
Would that grant an overrun?
I can see this going a couple ways:
A) No Bullrush as the enemy is larger than one size category over your size. This is the simplest ruling, but violates the Shield Slam's wording of Any.
B) Bull Rush, but no overrun. This seems to be the most accurate based on wording as the Shield Slam wording seems compulsory, yet the Breaker Momentum seems optional.
C) Bull Rush and Overrun. This throws all size category considerations out, allowing the seemingly compulsory Shield Slam to bull rush, and then taking the optional check for overrun
I've been arguing about something very like this on another thread.
People have been arguing that you don't get the bonus for Improved nor Greater Bull Rush when you Shield Slam because the Attack roll is not a Combat Maneuver Check.
People have been arguing that you don't get the bonus for Improved nor Greater trip when using Tripping Strike because you use the Crit Confirmation Roll and not a separate Trip Combat Maneuver Check.
But the only place where I have been able to find the Size Restriction on Tripping and Bull Rushing is under their Combat Maneuver descriptions. It's important for all of us to look for it somewhere else, because there are consequences if the restriction can't be found anywhere else.
It seems to me that if you are Tripping or Bull Rushing in some way that is not a Combat Maneuver, so much so that you don't get your bonuses for your improved and greater bull rush and trip feats, then you also don't get the penalties or restrictions, and that opens the door for Halflings Tripping and Bull Rushing Iron Golems and Great Wyrm Dragons.
It seems to me that you can't have it both ways. Bull Rushing with Shield Slam either is a Bull Rush Maneuver or else it isn't. If it is, I get my +4 Bonus for Improved and Greater bull Rush. If it isn't, then my Halfling can Bull Rush an Elephant, and he doesn't suffer the -2 Size Penalty for being small.
I am already turning in my head a PFS build. You will tremble when you hear this mouse roar.
You can bullrush a larger opponent, but they can't be more than one size category larger than you. With that said I think shield slams assumes the opponent will be within the correct size range, but it is open to interpretation.
If it does bypass the difference in size rule then if you successfully make the bullrush check you would also get a chance to make teh overrun check.
PS: I also see the shield slam as optional. Otherwise it is not a good ability to have. You can bullrush someone out of your attack range and if it is a large creature and you are medium then you are only helping him since he can full attack, and then 5-foot step away.
D: You can bullrush the opponent "who is no more than one size category larger than you". If the bullrush is successful then you may choose to make an overrunn attempt.