There is no file size or length limit because Smash modding uses a tool called ARCropolis that prevents file sizes from being an issue. I've modded in 30 minute long songs into Smash with no problems.
Volume control is just a simple slider in the program; the default volume for all songs is 2.7, and the current max volume is 20.
The interface for adding songs looks like this:
Where you define what game the song is from (you can even add a new game to the list if the song comes from a game that is not already covered by Smash, as I have done here), if the song is a remix or not, and of course, the name and copyright info of the song in the middle there.
The righthand category is the aforementioned volume slider, the "Sound Test" checkbox determines whether or not the song will show up in the sound test, or if it'll only appear when you're choosing the music for a stage before the match, and the "Special Category" is for defining two things.
The first option is "pinch songs", which is basically stuff like the Street Fighter stage music where it gets faster when the match is almost over. The other options are "Persona 3/4/5 Stage" which basically determines what form Mementos will take if this song was to play on that stage.
The interface for adding songs to playlists looks like this:
On the left is all the songs you've added to the game, and on the right is the playlist the game uses for the stage(s) in question. You just drag and drop from the left-hand column to the right, and the songs will display in the stage's My Music tab in that order.
The sliders you see on the right-hand column are for "incidence", which basically means how likely these songs are to play on the stage if you choose random music. By default, all added songs at set to 0 incidence because the game obviously doesn't have preset values for them, but you can change them both in this program and in-game with no issues.
Currently, the only thing we know how to add is music. Stuff like Spirits and stages are beyond us right now.