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.