Everything you read in books or see on videos describing how to tie a non-slip loop knot tells how to close the knot down before you tighten it so it will be a small loop. Does it really matter? I know it would not make sense to have a loop a foot long for a size 18 fly, but does it matter if the loop is 1/4" or 1/2" or 1" or 2"? There will still be two sections of tippet coming from the hook eye versus just one plus the knot on a non-loop knot, such as a clinch knot, and any size would still let the fly have movement.
Joe