To begin, make sure you watch this comprehensive video provided by NYSSRA.
Boots are the one piece of equipment that is directly attached to your feet and has to done right. Ideally, your boots and your feet should be one of the same and work in unison. This aspect is more important when they are used for racing.
The following brief guide is further provided for your reference. Visit a professional boot fitter should any issue arises.
The Mondo point convention is used throughout the industry. Aside from offered features, the biggest difference from make to make and model to model is the overall inside width and volume. Mondo sizing remains pretty much consistent. Use the following chart for general size reference. Expect ski boots to be fitted 1 to 2 sizes smaller as street shoes are typically over-sized for various reasons.
To get a good estimate for the size needed, just trace the skier’s (bigger) foot and measure it in cm from end to end. That measurement would be the Mondo point that is very close to his actual size with some room for growth.
To size the boots yourself, start with pulling out a liner for shell fit. With tips of toes touching the front, check for no more than 2-finger thick room in back of the heel (1.5 is ideal). Next, place the liner back into the boot. With everything buckled, bend your knees and ankles into a normal skiing stance, check for toes pulling back from the front. Lastly, check for any movement in the toe box and in the heal (upward and sideways). Your feet now should feel snug but not painful. If any slop or discomfort exists, try another pair.
A new pair of boot liners (especially an entry-level one) will pack half to one size. Used boots should be perfectly snug since another skier/racer will have broken them in.