I beleive that during the regular season, boys' high school varsity
basketball teams are limited to a roster of 15 players.

However, at my sons' school and at others I have noticed that rosters
have been expanded to 20+, with players added from junior varsity and
other lower level teams. And, these players even get into games and
score.

So, the question: What is the roster size for varsity teams in the CIF
Sectional and State Championship Playoffs?

Hello.

The 15 player maximum is in effect for both the sectional and state
championships.

From CIF's Advance Information Handbook: 
"Squad size is limited to 15 players in uniform, one head coach, two
assistant coaches, and one trainer."
Source: Page 7 of CIF State Basketball Tournament: Advance Information
Handbook, hosted by CIFstate.org
http://www.cifstate.org/Basketball/Basketball%20Handbooks/statehand.pdf

There are five or so separate handbooks for the different sections. I
checked  all of them and the 15 player limit is mentioned in every
one.  I notice that some of the handbooks mention that if the school
had more than 15 players on the roster during the regular season, it
would be possible to purchase additional medals for the extra players
at the school's own expense. Clearly, though, those additional players
would not be allowed to play in the games. The various sectional
handbooks are available on CIFstate.org:
2003 Basketball Handbooks and Forms. 
http://www.cifstate.org/Basketball/Basketball%20Handbooks/main.htm

search strategy: cif, basketball

I hope this helps.