Try the hat of disguise, or the spell disguise self in order to seem up to 1 foot taller (that is quite a bit taller for humans).
Alter self (lvl 2) is really polymorphing, so this is not what you want, but would work as long as you stay "Medium", which can be quite big.
You could always just use the disguise skill to be larger. You need only be passingly good at it and don't draw attention to yourself or your size. You could fake being a bit larger (a foot or so). Or you could really be Large by taking -10 and walk on stilts.
To minmax Disguise, you have the Keeper of the Veil regional trait to make it a class skill.
Or take the Cosmopolitan trait feat to do the same, but for two skills.
Casting disguise self grants a +10, but it lets you only be a foot taller, so maybe you'd really need a high skill rank as well.