Icelandic Horses have a wide range of sizes for their heads. I have three Icelandics and each one takes a different size halter, fly mask, and bridle. One takes an Arab size, one takes a horse size, and one takes extra large size!
The sidepull that we had made, specifically for Icelandics, accounts for the shorter length in the head, but the wider brow. The larger heads and some of the normal sized heads will generally take a larger brow band; my larger Icelandic uses a 17" brow band.
That is a very big subject. A lot depends on the shape of the horse's back, the length of the
horse's back, the width of the ribs, etc. See the following pages: