Currently you can enter a prefix of enormous length (100 characters) e.g. "phpbb_breakingallthemods_"
The user might think not expect any issues. But if a user uses such a prefix he can not install mods with indexes, as they are limited to 30 characters and there prefix already has 26, so there are 4 characters for the table and index name...

Therefore we should limit the length to 6 (phpbb_) or something like that.
This would also be cool for MOD-authors, because you can than use 30 - 6 = 24 characters for the table+index name and dont have to worry whether there are some users that need shorter names.