The rules do not define that
The text that SSD's answer refers to on p. 68 DMG says:
Demiplanes are extradimensional spaces that come into being by a variety of means and boast their own physical laws. Some are created by spells. Others exist naturally, as folds of reality pinched off from the rest of the multiverse. Theoretically, a plane shift spell can carry travelers to a demiplane, but the proper frequency required for the tuning fork would be extremely hard to acquire. The gate spell is more reliable, assuming the caster knows of the demiplane.
A demiplane can be as small as a single chamber or large enough to contain an entire realm. For example, a Mordenkainen 's magnificent mansion spell creates a demiplane consisting of a foyer with multiple adjoining rooms, while the land of Barovia (in the Ravenloft setting) exists entirely within a demiplane under the sway of its vampire lord, Strahd von Zarovich.
The the lower end of the size dimension range given for a Demiplane is as small as a single chamber, and that would be a lot larger than the extradimensional space needed to contain a tiny creature, or created by magic items like a bag of holding or a quiver of elhonna.
Also, you can use plane shift or a gate to travel to a known Demiplane. This would not work if the demiplane was tiny, too small to actually contain a small or medium-size spellcaster.
Not every extradimensional space counts as a demiplane, only those large enough. Mordenkainen's magnificient mansion, that is cited in the example, is large enough to be considered a Demiplane.
The pocket dimension of a familiar however has no defined size. It could be tiny too, or it could be a larger space allowing the familiars spirit to roam around. Depending on how large it was, it would just be an extradimensional space, or could count as a demiplane.
There is no danger of opening any planar rifts with it. The magic items that can cause such effects, such as a bag of holding or a portable hole spell this out explicitly. Find familiar has no such language. As spells only do what they say they do, there is no risk.