I found this very interesting article on how to figure out your perfect fit:
Your triathlon cycling shoes are the most important point of contact between you and your bike. It allows the transfer of power from you to the bike. Triathlon cycling shoes are essentially modified cycling shoes.
<!-- start: shared_blocks.36787051#below-paragraph-1 --><!-- end: shared_blocks.36787051#below-paragraph-1 -->The most important factor is getting the right fit.
They should feel very snug, allowing pretty much no movement of your foot in the shoe – though not so tight that they pinch or you lose circulation!
Any movement of your foot in the shoe means you lose the transfer of power.
The symptoms of cycling shoes being too big are actually similar to those you would expect to see from a normal shoe that is too small – rubbing, blisters and so on. If the shoe was smaller and movement was restricted you wouldn’t have these problems.
Because
getting the correct fit is so important, you should aim to try on cycling shoes at the same time of day as you usually ride. This is because feet tend to swell throughout the day so if you usually ride first thing but buy a shoe you have tried on in the evening, it may end up being too big.
Different brands will fit differently, for example some are narrower than others.
When you try your bike shoes on, don’t stand up and walk around in them. This is not what you will be using your shoes for, the weight distribution will be different to when cycling and so they will feel too small and uncomfortable.
So sit down, and check you have
no movement of your feet, other than possibly a little room to wiggle your toes slightly. Try to move the heel cup of the shoe – it shouldn’t move at all – otherwise it will do so when you ride, and you will not transfer power efficiently, and probably end up with blisters!
Triathlon cycling shoes are generally worn without socks, so bear this in mind when trying them on.
Another key feature of cycling shoes (both triathlon and cycling shoes in general) is the stiffness of the sole. A stiff sole allows effective transfer of power to the pedals. A carbon fiber sole is stiffer and lighter than molded plastic, and will last longer.
There are links to some brands and more reccomendations, to read the full article go here:
Triathlon Cycling Shoes - How To Choose The Right Ones.
