Ok - first off - Cali stingrays tend to be cool water rays (generally need water temps below 72F). So most tropical fish are pretty much eleminated - unless you can find species that do well in water temps in the range of 60's to low 70's.
As for tank size - yeah a 300 gallon is bare minimum for a Cali Stingray - a 360 gal (8'L x 3'W x 2'T) is even better.
A much better alternative to the cali stingray is the Cortez Stingray aka known as the Chocolate Chip Stingray. These tend to be a bit smaller (about 30-35%), and like tropical waters(72-82F). they also can be kept in a smaller tank - usually at least a 210-240 gallon (7-8'L x 2'W x 2'T).