Heel: When it comes to heel fit, it should fit snug, but not to the point of too restrictive or tight. Some small heel movement is fine, but it shouldn’t be to the point of sliding.
Instep: You shouldn’t feel any tightness or pinching in your upper. However, it should definitely be secure, so that you aren’t slipping.
Width: To have the correct width for your feet, it shouldn’t be too cramped, so that your feet or toes cannot properly splay. In essence, you want to have a little room for movement from side to side. Or at the very least, a feeling of security without discomfort of any sort.
Length: It’s important to remember that your feet will swell as they become heated from a run. With this in mind, you may go up a little in your shoe size compared to what you would wear for an everyday pair of shoes. To make it simple, you want a thumb’s width amount of space from your toe that is the longest to the front of your shoe.
While there are a lot of pointers to offer you in terms of fit, these above tips should get you started in the right direction.