What's the maximum key size or index size?
Since keys use indexes, the size of key is limited to maximum size of the index. For Firebird 1.x the maximum index key size is 252 bytes. For Firebird 2.x it's the quarter of the page size (1024 bytes for 4k page).
The computation is not too simple, but you can use this on-line calculator by Ivan Prenosil:
In case Ivan's site is down, there's a copy here at Firebird FAQ: