http://www.united.com/page/article/0,6722,50773,00.html

Cost*
Travel before May 5, 2008:
Within U.S./Canada: $85.00 USD
Travel on/after May 5, 2008:
Within U.S./Canada: $100.00 USD

Maximum size/weight
50 pounds/62 linear inches (W + H + D)

That's for domestic. Here are the international policies: http://www.united.com/page/article/0,6722,52482,00.html

Again, you'll be running into the 62 linear inch limit. It's very, very difficult to pack a bike under 62 linear inches, if it's even possible.