Airlines should have info including dimensions on their websites (quick Google for "British airways stroller" and it popped right up). Some airlines will list specific brands and models of car seats allowable on board. Be sure to print out all the regulations from each airline's website as time and time again, I and others I know have run into crews that are unaware of their own airline's rules regarding car seats and have wrongly confiscated them. I warned a coworker about this and she disregarded my advice. She ended up crying over having to hold her hefty baby in her lap on an international flight even though she was within the rules with a paid seat and carseat that was within regulations.
if there is room for interpretation or ambiguity in the regs, email customer service ahead of time and print that out. I am not exaggerating when I say cabin crew all too often do not know their own airline's regulations and will overzealously and erroneously enforce their personal interpretation of what the rules are. Personal experience as well as that of friends and coworkers.