Found this on United's website.
For most international travel and for travel within the US and Canada, you may travel with two free checked bags.
Each checked bag must have the name of the person travelling available on the outside of the bag. We also recommend that you place your name and address inside you baggage. Lock your bag and remove or secure hooks/straps.
You may substitute bowling, fishing, golfing, shooting, or skiing equipment for one piece of the two allowable checked pieces. Military personnel may substitute one duffel/b-4 bag free in lieu of their largest checked piece. Miscellaneous items such as sports equipment, live animals, musical instruments, extremely fragile or bulky items and non-sporting firearms and ammunition are subject to specific conditions, limitations and charges. Some may also be exempt from out normal size and weight restrictions. United is not responsible for cameras, VCRs, computers and any other electronic equipment or components, jewellery, cash or other similar valuable items contained in checked or unchecked baggage.
Valuable items, as well as medications or prescriptions, should be carried personally by you.
General Limitations
Of the two bags your ticket allows -
The largest checked bag must have linear dimensions that do not exceed 158 cm (62") and must weigh no more than 32 kg (70 pounds).
The second checked bag must have linear dimensions that do not exceed 140 cm (55") and must weigh no more than 32 kg (70 pounds).