This is probably the biggest issue for all weight bearing activities! They need to be right on many levels.
If the fit is too short- then the toes will blister, use the thumb at the end of the longest toe, widthways (not necessarily the big toe) and this is the gap that is needed from the toe to the end of the shoe.
If the fit is too wide – the foot will move around in the shoe too much.
If the fit is too narrow – they will cause pressure hotspots on the sides of your feet.
Wear the shoes well in advance of an event, do not wear shoes that are new to an event, make sure that they have been worn for at least two weeks before, and preferably for the time/distance your event takes. Do not wear shoes that are past their best (trainers usually are good for 350-450 miles before they start to lose their shock absorption and support) as that alone can cause your feet to be put at an angle that could lead to injuries and blisters.
You may need a bigger size shoe for your event day especially if your feet are prone to swelling during an event.
Fit Engo patches to the areas of the shoe that may be causing hotspots.