I get a lot of questions about how foot-shaped shoes should fit and feel, and it’s totally understandable. Since 98% of shoes are not actually shaped like healthy human feet, it stands to reason that the rules for fitting would be a bit different for shoes that are.
Of course, much of this will also apply to traditional shoes as well.

Length
The first thing that you want to do when you put your shoes on is kick your heel to the back and check your length. Stand with your knees bent, and you should have a full thumbnail width, or about 2/3 of an inch between your longest toe and the end of the shoe. Those with extra long toes can go a bit closer, while those with extra short toes should have even more room. Fitting this way will ensure the ball of the foot, flex zone, and arch are all in the right place.

Lacing
As you lace your shoes up, keep in mind your activity and terrain. For example, I lace looser for long climbs, and more snug for technical downhill. A general rule is you want to keep your shoes as loose as makes sense, since tight laces reduce blood flow and foot function, and can even lead to injuries.
For typical use, kick your heels back and get the laces finger tight over the forefoot and midfoot—it should be loose enough that you can slide a finger underneath through these areas. At the top of the shoe, pull the laces to be comfortably snug, making sure not to be too tight. Those with wider feet should custom lace to skip laces around where the foot is wide, like in this picture. The same idea can be applied to other parts of the shoe, such as over the top of the instep, where a lot of women feel their shoe binds.

Slide On/Off Lacing trick
For tame runs, walking, and casual use, use this trick to be able to slide your shoes on and off without having to tie and untie your laces.  Once you’ve put your foot in your shoe (unlaced), pull your heel back out almost all the way so it is barely resting in the shoe. Now tie the laces snug and slide your foot back in to the shoe.  The shoe will stay securely on your foot, but slide on and off with ease.

Feel
The most important thing is how your shoes feel. Once you’ve got them fit and laced up, they should feel similar to what you are used to at all the hold points from the ball to the heel. However, your toes should feel lost in space. We like to say, “If they don’t feel too big in your toes…they’re too small!” We know it will likely feel uncomfortably loose at first and might even drive you crazy. However, if you take between a day and a week to get used to the newfound room, blisters and black toe nails should be a thing of the past, and you’ll be adapted for a new level of comfort and performance.