There is technically no max party size, but there is a maximum number of people who can sit together at one table. Which is why larger parties are often told "your party may be split up."
And as for availability of larger tables, that is often limited, not just at CRT.
The majority of tables in all WDW restaurants (indeed, in most restaurants everywhere) are 4-seaters.
Next most common are 2 or 3-seaters.
And while WDW does have above-average numbers of larger (6, 8, or 10-seaters) tables, they still have fewer of those overall and so run out of them faster.