The diaphragm size of the mic will play less of a role in the recorded sound (an inexpensive one at least) than things like mic positioning, room sound (if any), eq/compressor settings etc..
Personally, if I'm after that kind of a smooth mellowed out sound I'd probably choose a ribbon mic or a darker condenser, but I've gotten good results over the years with a large number of mics.
If you have your heart set on a SD condenser though, the Audio Technica stuff is pretty good cost/quality wise (if you can find an older AT30xx on eBay or something they're excellent for the money). A lot of inexpensive condensers (small or large) suffer from high end harshness in my experience.
If you have a pre with a decent amount of gain, a dynamic mic (depending on the mic of course) can do a good job for this kind of sound as well.
