I find the most useful info. isn't at the school's individual website. If you want demographic/test/class size info. you should go to this page
http://www.montgomeryschoolsmd.org/schools/
and select the school you want from the "school-odex." Then, on the school's MCPS page, select "[school name] at a glance" and it will give a bunch of information, including class size. The MCPS page also has results of a parents' survey, but the response rate on those is usually small enough that I am not sure they are helpful.
For what it's worth, my child is also pretty shy and quiet and has done well at a school with larger class sizes.