Hi, it depends on the magnetic path length, and we must not have the gap more than say 10% of that generally.
Or say in an ETD core, you wouldnt want the gap more than 20% of the centre leg ferrite length.
But it depends on the problems caused by the leakage flux, which sprays out all over the place when the gap is big, and can cause heating in the coils of the inductor......so at high power the gap may need to be smaller so you dont overheat.
Though if its relatively low power , and the noise issue of the expansive flux field isnt a noise problem, then you can use any gap size you like , as long as you can achieve your inductance value with few enough turns....and with the wanted tolerance on inductance value...because the bigger the gap, generally the worse the inductance tolerance can be.
If you download standard ferroxcube core datasheets for eg ETD cores, PM cores, PQ cores, etc etc…you can see what size standard gap sizes they have off the shelf…so generally don’t have a gap any bigger than this, unless you have special analysis to say its ok.