Generally, the BB is designed to playback midi files, and, of course, using midi the tempo can be adjusted at will. User @AashiDeacon developed the method that I describe in the above tutorial, with which you can playback a wav file on the BB. I haven’t found a length limitation, per se, but the BB does have a 100mb limitation on the drum kit, so the custom drum kit containing the wav would need to be less than 100mb.
You can use any BPM wav soundfile, however, but its BPM will not be adjustable in the Beat Buddy. This is a “hack” to allow playback of backing tracks, so you lose some of the BB capabilities, for that song. The wav that you create would need to be 16bit 44.1khz, and you will need some type of midi creation software to create the BB .sng file with the one long note as explained in the tutorial. I use Logic Pro, but anything with a good midi editor, i.e Ableton, Cubase, Audacity, etc., will work.
Also note that the BB seems to have issues with some meta tags that get attached to wav files from some programs. There are tag stripper programs to get rid of those problems, but I find that NCH Software’s Switch is great for this, and it’s real good for changing files from 48khz to 44.1khz, or mp3 to wav, among other things.