It seem like it would be a simple thing to do. The filters now include:
- file extension
- words in the name
- maybe some other things.

But file size would cover a range of sizes .. a minimum size and max size.
Maybe it's too much processing for the hosts? Because each host has to handle many searches per second while he's using his computer to do other things.

But wait. The client could take the same results he gets now and filter according to file size, right?

But meanwhile you can order the results by the file size, so the biggest ones appear at the top.