A great alternative to knee high wellies, the Men's Country Cross Bottillon from Le Chameau is a comfortable and robust jersey lined mid-calf height wellington boot. Practical for a wide variety of uses.
Wearing what you would normally want to wear inside your wellies (trousers/jeans and thick socks, for example), sit on a chair with your feet flat on the floor.
Wrap a tape measure (or piece of string that can be measured afterwards) comfortably tight around the widest part of your calf, inserting either one or two fingers between the tape and your leg.
Take a note of this measurement, which will give you the ideal calf size for your wellington boots.