This thread has some helpful advice, including my own experience flying with a 24" x 36" painting without issue.

https://www.flyertalk.com/forum/alas...-question.html

I had no problems placing it between my seat and the window, while seated in first class (seat 2A) when the Airbuses still had the VX big white recliner seats in F, SFO-JFK.

One other consideration is if it will fit through the Xray machine at the TSA checkpoint @ SEA.

I think it would also likely fit in the coat closets in 737 F cabins.