1) Bigger is not always better. I paid $15 for a United Airlines bicycle box and thought it would be really good because it was huge. I was able to roll my bicycle in. But I've never seen such a badly damaged cardboard box as that one when it came off the plane at my destination. It was too big and didn't have the support and stability it needed.

2) First look up the bicycle baggage information which will be a different location on the website than other baggage information ... then look up oversized baggage information for the max size. Many airlines do have a max size these days.

For example, here's what it says on the United Airlines site regarding baggage size ... and believe me, some bicycle boxes are awfully close to that limit.
"Baggage in excess of 100 pounds (45 kilograms) or 115 linear inches (292 cm)(total length + width + height) will not be accepted as checked baggage."
https://www.united.com/CMS/en-US/tra...ageExcess.aspx

With Air Canada, anything over 115 linear inches goes Air Cargo ...
http://www.aircanada.com/en/travelin...e/checked.html

With Qantas, other baggage seems to be limited mainly by weight, but for bicycles ...

Qantas bike pack dimensions are:
Length: 140cm (55in)
Width: 30cm (12in)
Height: 80cm (32in)
http://www.qantas.com.au/travel/airl...lobal/en#jump0



So be sure to check. Read all the baggage information you can for the airline with which you intend to fly. And if you change airlines mid-flight, take special care because the two airlines could have different rules.