BA codeshare
 

Please excuse the simple questions, haven't flown many codeshares like this. I am flying MCO to LGW with an American flight #, but the flight is operation by BA. Will American's hand baggage size limits apply, or BA's? American's limits are a bit more generous. Also, at MCO do I check in at the American desk or BA desk? Thank you.

You always check in with the operating carrier of the first segment, BA in your case. And for on board things like hand baggage size limits, it's the operating carrier's rules that apply (again BA). But the checked baggage allowance should be based on the marketing carrier; the checked baggage allowance must by US law be listed on your eticket receipt, and whatever allowance is listed on your ticket applies (at minimum).