Pet carriers on international flights

I haven't seen this question asked yet.

I am doing some early research for travel from USA to Brazil. I am taking my 8 year old cat with me. This topic is in regard only to pet carriers in the cabin.

Each airline seems to have different regulations on the carrier size. (L x W x H)
- JetBlue: 17 x 12.5 x 8.5" (43 x 32 x 22 cm)
- United: 18 x 11 x 11" (46 x 28 x 28 cm)
- SWA: 18.5 x 8.5 x 13.5" (47 x 22 x 34 cm)
- Delta: varies depending on the flight and aircraft.

My question is, how strict are the airlines on the pet carrier size? Will they let me onboard with something "a few cm" too tall? Of course, it would need to fit under the seat. Assuming I could smoosh a flexible fabric carrier into position, has anyone had experience with this?

Thanks!

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