Say a medium-ish sized bag that weighs 50 lbs (23 kg).
There is no clear price information mentioned on the airport website. The easiest thing to do will be to call or email them. Their website provides this information:
Baggage storage is exclusively available at the Airport Travel Agency, located on the Departures/Ticketing Level of the International Terminal, near the entrance to Gates G91-G102. The Airport Travel Agency is open daily from 7:00 a.m. to 11:00 p.m. No reservations are required for baggage storage. All baggage is x-rayed prior to storage. Rates are assessed per each 24 hour period that an article is in storage (articles in storage for less than 24 hours will be subject to the 24 hour rate), and vary according to the size of the object. Please contact the Airport Travel Agency at 650.877.0422 or CF.Airport@gmail.com for additional information.
SF To Do says that the typical rates vary from $15 to $35 whereas the pricing on the Airport Travel Agency Website is quite ambiguous and seems to not agree with the same. I would assume that your size luggage would cost approximately $25 but the best solution is to just ask via email.
SFO offers only one staffed location where you can store baggage. Airport Travel Agency, located in the Departures/Ticketing Level of the International Terminal, is near the entrance to Gates G91-G102. It offers storage for $15 to $35 per day depending on the size of items you are storing.
They finally put the price online:
We have a five-hour layover before an international flight, so we can't checkin our luggage. Figuring with four checked-in bags and two cabin bags, we may end up with $170 luggage storage bill. Instead, we decided to rent a car to the city with our bags for $72. Beats being at the airport for 5 hours.