if you get a sling you can order a longer one. Its not so much about the strap as it is how the baby is in there.
You can do a wrap instead or a Mei Tai. Wraps are generally 3-6yrds long, can be longer for heavier people. MT are one size but the width of the panel and strap length can be extended for dads/ bigger moms.
I am a size 14 and id go with a wrap for a small baby and a MT when the baby got bigger. If BF, id do a sling though because you can use the strap for covering up
ETA: You can MAKE a wrap yourself! Literally go to the fabric store and buy 3 yards one way stretch cotton knit fabric and sew the edges if you want too and then there ya go. youll need the wraps measurements to be 3-6yards long by 22inches.