Like any cycling shoe, a close fit is essential. Any movement of the foot in the shoe means lost power and efficiency. Also, if your foot is moving in the shoe with each pedal stroke, that can easily lead to rubbing, soreness and blisters which won’t help your run split.
Unlike when you’re fitting a regular cycling shoe though, there’s a key tri-specific point to think about. The majority of triathletes tend to ride, specifically on race day, without socks, so this has to be taken into account when sizing up your shoes. I’d always advise trying on shoes in a shop rather than buying blindly online. But go shopping with the lightest and thinness socks you can find.
However, especially if you’re targeting long-course events, don’t go too tight, especially in the toe-box, and ensure that there’s scope for you to loosen off the fit as your feet will swell significantly over the course of a bike leg.
Finally, once you’ve got your shoe fit dialled in, pay similar close attention to your cleats as, if these aren’t set up correctly, they can significantly affect both foot comfort and cycling performance as well as contribute to injuries.