With the kneecap centered over the knee, the shin guard should extend down to the ankle. Bring your skates with you when shopping and wear them when fitting your shin guards for best results. Depending on the position you play, find a balance between bulk and sufficient protection. The rule of thumb is to cut your stick at nose level in street shoes. The height will be at the bottom of your lip when your skates are on.
Forwards tend to like their sticks a bit shorter, defensemen a bit longer.
While some players like to buy a particular brand, some brands tend to fit certain types of feet better than others. For example, CCMs tend to fit players with higher arches and feature a wider toe cap area. For those with fully-grown feet, a rough rule of thumb is, when fitting, your big toe should actually touch the end of the skate. Kick the heel back and lace up. After a few skates the heel will fully settle in, pulling back a bit for a perfect fit.