I believe it's limited by the total number of characters, not the number of items themselves (+ counts one comma between very option)? This is a quirk of how it was originally implemented, as an augmentation to the raw Laravel validation rule.
The only way to make it bigger would be to use the extension API to register a custom dropdown as its own new field type, or edit the database and validation rule to store longer validation strings. Things could break when doing the latter though.
Another option might be to use the Flarum extension API to hard-code a value in the database model every time it's retrieved, that way the long string can be stored in extend.php
rather than in the database column.