There is no minimum for the OTK DB size, so defaults 100MB is the minimum as far as I know.
Now, as for a formula or DB sizing, we do not have one, but you can check against the otk_db based on the number of tokens you store by running something like:
select (index_length+data_length)/table_rows rowsize from information_schema.TABLES where table_name = 'oauth_token' and table_schema='otk_db';
This will give you an average in bytes per token.
Now multiple that by the numbers of token you think to keep and you will have the minimum size for that table (oauth_token).
Do keep in mind that CA strongly recommend to run maintenance on the otk_db to avoid DB and size issues.
You can find more information on this in the product online manual: