David, To answer your question, no Clyde doesn't do custom shoes. He may be able to make the Shrew Shoe a little wider but to make it longer isn't possible without making a new form.
Design in a snowshoe is an important factor to consider when it comes to performance just like with bows. I have a pair of custom shoes made for me by Iverson a few years ago. They are the Ojibwa style and are 13X65". They are a very heavy shoe, probably more than twice the weight of the SS, with a lot of up sweep at the toe. The toe starts the upturn right in front of the foot hole and is 12" from toe to ground. Clyde's Ojibwa shoe leaves more of the shoe on the surface before it's upturn to 7" from the toe to ground. The Ojibwa shoe although bigger doesn't give me the support that the Shrew Shoe does.
Snow shoe weight is also an important factor to consider. Clydes shoes are made from Hack Berry instead of the traditional Ash. Clyde tested both woods and found that Hack Berry is 20% tougher than ash and is 1/2 the weight. The webbing on the Shrew Shoe is poly-lace webbing. Poly-lace looks like rawhide but is much stronger with less maintenance and "lighter" weight. With the Poly-lace it's possible to get a tighter for less sage and a tighter pattern for better flotation.
Here's the huge Ojibwa snoeshoes.