Actually, bike boxes come in sizes. The more compactly the bike fits into the box the better. Go to a bike shop and ask to look at a new factory packed bike in the box. Try to mimic that. EVERTHING the factories do when they pack a bike is for a reason. Even the gear the chain is in matters.

During the time that I owned my bike store I received over 1,000 factory packed bikes via UPS with zero damage claims.