The Class size row sits under Location on the class page. It shows how many people can book each session, for example 12 places. Until you choose a resource it reads Choose a room first.
Select the row to open Class size, enter the number of Places, and select Save. Leave the field empty to use the room's capacity; the row then says Same as the room. You can set a size larger than the room holds. Naayya warns you, for example This is more than Room 1 holds (20). Customers can still book every place., and the row shows the same warning.
To change one session only, open the session and click the value in its Places row under This session. The dialog is called Places in this session. Leave it empty to use the class's size; the row then says Same as the class.
Naayya uses the first size it finds: the session's own places, then the class size, then the room's capacity. Use this instead of adding a second copy of a room to get a different class size. Two copies of one room hide clashes between the classes booked in them.