i have regular sized locs, so i don't know if i'll be of much help. what ive heard about sisterlocks tho is that you have to care for them because they're so tiny and prone to interlocking with other locks, so you have to maintain the do. meaning you'll prob be in the salon up-keeping them a lot. that's one reason i didnt sisterlock cause i was scared lol. i saw a beautiful head in atl tho and it was so gorgeous. it looked like loose hair. but i like the low maintenance of my locks now so i know sisterlocking is something i could never keep up if i wanted them to be right.
you can get locks any size you want them. you just have to go to someone who knows what they're doing. and if you want your locks to lock fast, then you might be waiting a bit with sl hair because the longer the hair, most likely the longer the wait. i locked at chin length and it took about a month for them to start locking, but they were fully locked around the 3rd or so month.