I would like to be able to express the concept of circular or spherical size, i.e. radius or diameter. I'm toying with using mdi-google-circles, mdi-null, mdi-arrow-expand or mdi-arrow-expand-horizontal in the mean time, but I think that might be confusing.
A more clear icon might have a circle with double-ended arrow inside or outside (possibly with leader lines)

Another possibility would be to show circles of different sizes, e.g.

I'm not sure how to express this neatly in a "material design way", but I hope someone would be willing to take a go at it.
In engineering drawings this is often called out like this:


Given the context would the circle or circle-outline icons not communicate this?
@PeterShaggyNoble Thanks for the suggestions, but no, I don't think those are good candidates in this case. While they are circular, there is nothing about them that indicates which property is of interest either by variation (e.g. different sizes) or indication (e.g. arrow across diameter).
Plus this is in the context of a form, which makes them too easily confused with option/radio buttons or bullets/decoration. On that form there are also quantities related to concentration/density and volume.
I believe a simple outlined circle with a slash (which is the official drafting symbol) would suffice.

@MrGrigri Well, that's why I am using mdi-null for now. It is very similar. It just has the unfortunate side-effect of looking like a "no symbol" and, of course, null / empty set.
null is not a circle, and this one is.

I can add this one, but I think we should have a more representation of it also. Same for radius.
Exactly! I'll get these added tonight.
@MrGrigri thanks for all of your effort! diameter-outline looks exactly like what I'd like to use. My only point of feedback with these is that radius looks somewhat like a clock. I am not sure how one would go about remedying that though.
I don't because our clock has two hands. It could be confused with the timer icon though. However, context plays a huge role.
Added all 5. 馃樅
The will be in the 3.1.xx release.