A very plausible choice for when inflation might have happened would be when the energy scales of the universe were at the scale of grand unified theories... which unify the weak, strong and electromagnetic interactions into a single unified interaction... we're talking about energies which are about 1016 times the equivalent energy of a proton mass.... the initial patch would only have to be the ridiculously small size of about 10-28 cm across to be able to lead ultimately to the creation of everything that we see on the vast scale of which we see it.

Lecture 1: Inflationary Cosmology: Is Our Universe Part of a Multiverse? Part I. - The Early Universe (2012)