From a size perspective you may be ok (fee will apply though) - policy is here:

http://www.united.com/page/article/0,6867,52907,00.html

Highlights - maximum dimension of any side: 62 inches, maximum linear dimension (l + w + h): 115 inches.

I would assume it is ok from a weight perspective also.

The real question is around how it will survive the journey - if it's in its original packing then this will obviously help.

For customs - it will depend on the value of it. Do you know what this would be? Some details here: http://www.cbp.gov/xp/cgov/travel/va...aying_duty.xml