There are quite a few dim sum restaurants in Chinatown with seating, so you won't have any problem finding one. Are you looking for a large dining hall type of place, or are you more interested in food quality, as the two don't necessarily go hand in hand. Just to rattle off a few names there's Gold Mountain on Broadway, Don Hol and New Asia and Y Ben House on Pacific and New Hong Kong Menu on Commercial St. Lots of past threads discuss these and other dim sum options so use the function in the upper right hand corner of this page.
Y Ben House Restaurant
835 Pacific Ave, San Francisco, CA 94133
Gold Mountain Restaurant
644 Broadway, San Francisco, CA 94133
New Hong Kong Menu
667 Commercial St, San Francisco, CA 94111
772 Pacific Ave, San Francisco, CA 94133