Which paired meridians form a small circle?

If you are talking about geographic meridians on the Earth, there are none. Any paired meridians (the numbers, added together, total 180 degrees) will form a Great Circle, however. This includes the Prime Meridian (0 degrees) and Meridian 180; 90 degrees East and 90 degrees West; 115 degrees East and 65 degrees West--the list goes on and on.