15-20mmHg is considered a moderate amount of compression. This is the compression level most recommended for travel, standing or sitting for long periods of time, minor swelling and minor varicose veins. At this level socks are relatively easy to put on and very comfortable to wear.
Wide-calf sizes fit people with larger than average legs. Measure the widest part of your calf. If it's over 18", you will probably need a wide-calf size. (Check the size chart.) Most fit up to a 24" calf, Core-Spun is sized up to 25.5".