What would happen if a medium sized person used beastshape 3 to turn into a huge creature and then tried to reach into and antimagic field?
id say that if he uses the form's extra reach (beyond his medium size reach) then it can't effect. the part that go into the anti-magic is nulled back into medium.
But if it was within the medium characters 5 foot reach he could hit with a hand. Sounds reasonable.
yea, but the hit would be using his own attack not the beast form. so if he has no natural attack\ improved unarmed strike it might also provoke an aoo.
Yep. In the particular case I'm thinking of, it would be a grapple attempt. Using size and strength without the modifiers for the spell.
The rules don't say, but I'd say they get no benefits of the spell. Since you can't partially change shape it would be fair to say the spell stops working while you reach inside the field, but while your arm is outside of the magic area you immediately turn back into whatever creature the spell is allowing you to be.
Either way the intent of the spell is to stop magic so I'd make sure that is what happened.