The common rule for most pieces of safety gear is to aim for a comfortable snug fit which also applies for the fitment of knee braces. The padding for the hinge point which sits either side of the rider's knee joint should be snug too tight and the rider will experience pain when riding and too loose the brace may slip down the leg and not sit in the correct place on the knee.
Snug is best:
Most braces on the market come with adjustment options so replacing the padding can help you find the exact fit for you. To check the knee brace is not too loose put a set of braces on and walk around without any boots on, if they are loose they will slip down your legs easily. Also make sure you try both left and right sides on as some knees are different sizes left and right.