My recollection is that 15 mm is normally too big and 10 mm too small.
These hoses are generally reinforced so won't "stretch" when softened in
boiling water in the way garden hose will.
If you can find a suitable undersized coupling tube you can enlarge it
by winding pvc electrical tape on it until it is a good fit, then use
jubilee clips as you suggest.
If you feel brave, tackle one of the ends to free up the coupler, then
push this in the cut down hose and secure with jubilee clip. The risk
there is that the end fittings are usually plastic and there is a risk
of them breaking either as you disassemble (angle grinder) or when you
tighten the jubilee clip.
Because I don't throw anything away I have a collection of hoses off
used machines which are usually about 1.5 metres long. IME that's about
the minimum you want if you need to pull the machine out while it is
connected. Perhaps you can find a local squirrel?