What are the five largest high school marching bands in California?

This can be a difficult question to answer, not only because band sizes fluctuate from year to year, but because some bands only march upperclassmen and/or hold out alternates.

Arcadia HS from Arcadia is usually the biggest, ranging between 375 and 425 members.

Reedley HS from Reedley is often close behind, with between 275 and 350 members.

James Logan HS from Union City is frequently between 225 and 275 members.

After that it gets tough to figure out, as there are dozens of bands in California that routinely march around or over 200 members. An incomplete list:
Analy HS, Sebastopol
Amador Valley HS, Pleasanton
Ayala HS, Chino Hills
Diamond Bar HS, Diamond Bar
El Dorado HS, Placentia
Foothill HS, Pleasanton
Homestead HS, Cupertino
John F. Kennedy HS, La Palma
Monache HS, Porterville
Mt. Carmel HS, San Diego
Porterville HS, Porterville
Poway HS, Poway
Rancho Bernardo HS, San Diego
Saratoga HS, Saratoga