Is it not listed on your itinerary receipt? Here's an example from one of my receipts:
Baggage regulations
The following baggage rules apply per passenger:
in Economy Class:
Weight / Pieces
Dimension (width+height+depth)
Fee
1st checked bag
up to 50lbs
up to 62
free of charge
2nd checked bag
up to 50lbs
up to 62
USD 100.00
Carry-on baggage
max. 1 piece
up to 45
free of charge
in Business, Business First and First Class:
Weight / Pieces
Dimension (width+height+depth)
Fee
1st checked bag
up to 70lbs
up to 62
free of charge
2nd checked bag
up to 70lbs
up to 62
free of charge
Carry-on baggage
max. 1 piece
up to 45
free of charge
For this journey, the baggage rules of United Airlines, Inc. apply.
Over and above the regulations displayed, which may already include status-customer benefits, other status-dependent benefits may be granted. These will be granted at check-in after validation.
Please be advised that due to security reasons items which are classified as dangerous goods are prohibited on board.