Hello,
I'm flying Corpus Christi - Houston // Houston - London Heathrow soon.
I need to take a lot of things with me for a relative. The maximum luggage size allowed is 62 linear inches (width+length+height) which could be a box of 27x18x18" (68x45x45cm). I find this rather small.

Has anyone here got some experience with how strict UA is with luggage sizes?
I'm thinking of taking the risk and getting some bigger boxes, but want to hear your opinion on the chance of being charged for oversize luggage.
Oversize luggage costs 200$ per piece, so it's very expensive. I would have a friend bringing me to the airport and if they don't accept the large boxes I would have to re-pack in the airport into smaller boxes and leave some stuff behind.
So, what do you think what they would accept as normal luggage?

Thanks for your help!