Well, a medium light shield would bash for 1d3 damage while a medium heavy shield bashes for 1d4 damage. If you use the standard size step-up, a large light shield is treated as 1 size larger (1d4) damage, but takes -2 to the attack roll. Then, you need to consider whether it also counts as a heavy shield for +2 bonus to AC or if it still gets only +1 bonus for a light shield. Lastly, what, if any, penalties would apply to using oversized armor (since shields straddle the line between weapon and armor). I'd presume that, if allowed, it would involve a higher skill check penalty and arcane failure rate. Possibly also a penalty to max dex to AC.
I figured the damage out myself using that chart from my last post. According to the chart Colossal is 3d6 damage for a maximum of 0f 18 damage. If following the formula, Colossal+/Titanic would be 4d6 for a maximum of 24 damage. 24 divided by 3 is 8 which means it can be simplified to 3d8 damage.