From my experience, it really depends on this context questions:
- does it need to be interactive (touchable, clickable)?
- does the visibility of the face photo is important in that context?
- do you have an existing framework (dev or design)?
Depending on that, you would probably have different sizing needs.
For interactive avatar
The kind of avatar I can click/touch to do something on the interface (see profile, edit my avatar, etc.) you'll need to comply with the minimum size for a touchable interface component: Apple and Google don't agree here, but around 48px wide is enough.
If you don't need this to be touchable, do you have a context where the face of the people, or their avatar, matters? If yes, you would rather take a higher sizing. If not, a small one can be enough.
As @AndriiD said in their answer, most Designers use a 4/8px grid, so using a size among 24px (min IMO), 32, 40, 48, etc. is a safe solution. But, regarding my last point, if your team is using another framework, it could be a 5px grid for instance. If I remember well, Boostrap is using a 5px grid.
In your case
I would go for a sizing that is close to the chips used in Material UI. They are purely indicative avatar to give you more metadata on a card or on another component.
The chips uses an image size if 24px.
Hope this helps 😊