Welcome to Enworld!
So, let's recap:
Druid Wildshape imitates the Beast Shape spells. That is a magical effect that changes your size, among other things.
Righteous might is a buff that increases your size and does some other nasty things.
This is the spell description for RM:
http://www.d20pfsrd.com/magic/all-spells/r/righteous-might
On the last paragraph, it says:
"All equipment you wear or carry is similarly enlarged by the spell. Melee weapons deal more damage (see Table: Tiny and Large Weapon Damage). Other magical properties are not affected by this spell. Any enlarged item that leaves your possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown and projectile weapons deal their normal damage.
Magical effects that increase size do not stack."
Emphasis mine.
BUT (and that's a big "but"), it says "Magical EFFECTS" that increase size do not stack, so we might argue that all other "effects" of each spell do function simultaneously. Bear this in mind on your next session (Oh, snap!).
***BONUS POINTS***
A character CANNOT be affected by two spells with the polymorph descriptor at the same type. Beast shape has the descriptor, but Righteous Might doesn't. Keep this in mind, as it might come in handy in the future.