Too small and you're cramped and uncomfortable, too large and the pad can slide down the arm. Be sure there's a donut in the elbow area. If too many straps leave you feeling constricted, there are models available with no middle strap in the inside crease of your arm, and some with no middle or upper straps. More and more companies are offering models with forearm slash guards as well. If the pants are too large it will interfere with your shin guards and inhibit your skating.
Most pants come with belts so they can be used without suspenders, but belts can be constricting.
Many players are going back to suspenders for a free-flowing feel. Depending on your preference, the upper part of the glove can extend up the forearm to the beginning of the elbow pad. Gloves should be comfortable and somewhat loose. 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.