When adding in new tables, you need to fill out/edit the 3 different capacity columns:
Min People - This is the smallest group that may be assigned to this area.
Max (seated) - This is the smallest group that may be assigned to this area if everyone is seated. (NB by default this is what the 'Auto-Confirm' function runs off, however, you can specify within a booking type if you want auto-confirm to run off the Max People capacity)
Max People - This is the largest group that may be assigned to this area if everyone is seated and standing. (NB Collins will indicate in the diary with a red highlight if this capacity is exceeded).
If you would like to create and edit zones for your tables, here's an article to show you how.
IMPORTANT - along with your Opening Times, your Tables/Areas section essentially creates your Diary.
To see a video tutorial on setting/editing table capacities, please click here and watch from 01:42 - 02:33.