qBittorrent official forums

I would like to see an option in which we can set how many files within specific torrents can be downloading at once. I personally like being able to download all the small files first, medium second, and large last.

The bittorrent protocol has no concept of downloading 'files', it downloads pieces, and a piece may include several 'files', one single 'file',  or 'parts' of more than one file.

set how many files within specific torrents can be downloading at once

You can already do that manually in the 'content' tab of the properties, you 'know' what files are, qbittorrent/libtorrent can work out the relevant piece numbers.

AND: At creation of the payload 'files' are ordered with the largest first by default, this reduces the number of 'cross-file' pieces that are needed.