Flying Copa with a small dog – tips for carrier!!!

Hi! I’m flying JFK → Panama → Montevideo roundtrip with my Cavapoo (she’s ~12 lbs) on Copa. I’ve never flown internationally or domestically with my dog before and would love some advice.

• My layover is in Panama and only about 1.5 hours. How do people handle dogs not having indoor relief areas during layovers? During security, do I need to take her out of the carrier, or stay with her the whole time? -for my carrier, I’m undecided between Sherpa Medium and Large. The Large would give her more room, but it’s slightly bigger than Copa’s under-seat dimensions. I’m not sure how strict they are. Any advice or experiences? -Any tips or anything that made international flights with a dog smoother on Copa?

Thanks in advance for any advice!