Normally the woven straps (Cotton, Polypropyln, Nylon, etc.) are equipped with a glide buckle. With the glide buckle it is easy to adjust a guitar strap / bass strap.
The most of the leather guitar straps can be adjusted to the desird length by use of adjustment tab and fitting tube holes (see pictures below).
Due to simplicity I used the plain pictures from Levy's Leathers. Of course the descriptions also apply to the guitar straps from other producers.