You need to have an idea of the size feet of your recipients, then check this to judge the length of said shoe size. You can knit the clogs according to the size in the pattern (women's 6, 8, 10, men's 11, 12), then as you felt, measure the length until they are the approximate size...... (hint: felt in a pillowcase tied at one end to save your washer from too much lint - and there will be lots of it).
This is just theory on my part, having knit these only for my DH and myself so far.
Have fun - these a fast knit and come out great! What yarn and colors are you using?