Some letters, some blanks.

Type the ones you know and leave the rest as underscores. The length of the pattern is the length of the word.

Underscore, ? or . for a blank. Reading as

Every length, ready-made

A pattern of all blanks is just a length, and those already have pages:

Questions this page gets asked

What counts as a blank?

An underscore, a question mark or a full stop — all three do the same thing, because people type all three. Everything else is taken as a letter that must be in that exact position.

Does the length matter?

It is the length. A pattern of five positions finds five-letter words and nothing else; add or remove a blank to change the length.

Is this a crossword solver?

Yes, for the part a crossword actually gives you: the length of the answer and the letters already crossed. It knows nothing about the clue.

Which words can it find?

The 172,823 words of the public-domain dictionary this site is built on. It holds no tournament word list, so a word valid in a club game can be missing here.