For maximum pedalling efficiency, the treadmill must fit perfectly. It doesn't twist at the heel when you pedal, and you feel comfortable in it. A common selection mistake is that cyclists buy unnecessarily larger treads. A road cycling tread has a hard sole that doesn't bend. Thus, compared to regular walking shoes, there is no need for extra space in the cycling treads. They should be just right.
Fi'zi:k also offers selected models in wide versions, i.e. with extended foot space. So there really is something for everyone.