These are usually either condenser microphones or ribbon microphones, depending on the sound you are going for.
Condenser mics will create a more accurate depiction of the string tone. Ribbon mics, however, will add warmth and extra character to the sound at the expense of accuracy.
If you were to use a condenser, the size would depend on what you were recording. Violin will almost always be a small diaphragm condenser. Cello will usually be a large diaphragm.
If you are recording an entire string section, it’s best to stick with a pair of large diaphragm condensers. You’ll be able to capture a much larger part of the frequency spectrum.