There is a comprehensive study of the barred surf perch (Amphistichus argenteus) done by the Cal DFG years ago, which tells you a lot about the barred surf perch. One of the graphs in the article correlates size with length. (See below) The study is here: It has pictures of the study, which are kinda cool, like this one:
Anyhow, the info shown below would indicate that an 18inch BSP would be a really big one. It would suggest it's much more than 3-4 pounds. The largest BSP in the table is 354mm (~13.6 inches) and it would weigh 1.45 kg or 3.19 pounds. An 18 incher would be massive.