From their web site:
Luggage Policy on Route KX
SamTrans is unable to allow excess luggage to be carried on Route KX between San Francisco and the San Francisco International Airport in either direction.
SamTrans Carry-on Policy States:
"If a passenger can carry the item(s) on his/her lap or store it under or next to him/her on a seat unneeded and not hurt anyone else, and not interfere with others, the item is probably safe to transport." The final decision will be made by the bus operator, who is responsible for the safe transport of everyone on the vehicle.
Alternatives & Information:
If you are unable to ride Route KX, Route 292 can take you to The City or the airport.