Archer's right, that it's a compact 1911 style pistol. The thing to be careful of, though, is that SIG put their own touches on the slide contours for their 1911 design. At least, I know they did on the full-sized GSRs, so I am assuming that is true for the C3, as well.
For the 5" models, the rule of thumb seems to be that leather holsters for 1911s will fit the GSRs because there is a bit more give; kydex holsters will not in general fit unless they are specific to the SIG design. The C3 does not seem to have a rail, so you need not worry about that detail at least...
I'd say find a leather holster maker who will let you return if you have problems and try an officer sized 1911 holster.
The top end is Commander size - that is what goes in the holster. The frame is Officer's size, which is what makes these ideal carry guns.
If you have found one at a shop, grab it. If you anticipate ordering it, be aware that I have been waiting six months for an RCS (same as a C3, but dehorned in the custom shop). Not saying you'll have to wait too - since hopefully they'll be doing a production run soon, but with Sig, you can never tell.
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