Pain in the right arm when playing guitar while sitting. Help, please!

[s2y]

I have to utilize classical posture since I'm so small.

Beveled edges can also help. I use them on big and/or deep acoustics.

[Scotso]

It sounds like the edge of the guitar is either cutting blood flow or is impinging a nerve in you arm or both. The recs to go smaller or try a more classical guitar position are solid.

[printer2]

Might help.

[rmp]

like Eric says,, get a Neckup Strap! it's a game changer.

[Benjo]

I had to sell a jumbo size guitar bc it was giving me the same problems the OP describes. Body shape is a really important consideration in terms of comfort and ergonomics. If your guitar is big, I'd start there, along with using a device that anchors it on your left leg. Using a foot stool is not optimal since it raises one leg higher than the other.