In my experience as a science editor, scholarly and scientific publishers I've worked with restrict the use of "circa" to years or dates: circa 1940, circa 3500 BCE, circa 250 MYA. In other contexts "approximately" or "about" or the approximate symbol (tilde) "~" is preferred: approximately 170 hectares, about two weeks ago, ~50 ft.

As far as I recall, I too have only seen it used with dates.

Me too, or neither as the case may be.

Certainly any editor is free to make any sort of style decisions regarding the scope of any term such as circa. The B&M OED notes that, as a prepostion, it is often used with dates. It also notes that it is used as a combining form, citing an example in circa-continental. However it simply means 'around, round about, about' and, as such I am sure it has been used more generally. I see no reason, in any linguistic sense, why it shouldn't. The first ten hits for circa acres include five for uses where the acreage being talked about is approximate.

Oh, I thought it was circum-continental, tho' I have never seen it in writing to check.

Circum-continental is the singular.

what a noisy hyphen.

what a noisy hyphen

"Do you hear the hyphens, Clarisse?" And what about the em dashes?

just a smidge, or perhaps a pinch.

FWIW, the Latin preposition circa (which governs the accusative case) is not limited to being used with time. It can mean 'around, among, surrounding; escorting (of persons); near, neighboring; nearly, almost' etc.

In other words: 'circa everything!'

But what about the English preposition circa?

M-W says 'used especially with dates', not exclusively! link

"Management structures and the infrastructure are in place to drive a sustained step up in store openings and activities with the opening program rising from circa 10 outlets per annum to circa 30 over an unspecified time frame — we assume by 2013-15," Black said. link

-ron o.