Get someone with experience to help and teach you about properly fitting yourself and the horse with the tack. To explain on the internet may create confusion or discomfort for both you and the horse.
There's more flexibility with bridles because they can be adjusted, and like what others have said, they generally come in size pony, cob, horse, and draft. You will need to know the size and type of bit that the horse will need and it is best to have someone help you with that. Having an incorrect bit could cause considerable problems and jeopardize safety. After you have the appropriate bit and bridle, you adjust it so there are two wrinkles on each side of the mouth. You don't want it too loose nor too tight.
When it comes to saddles, you need to know the tree/gullet size and seat size. Again, have an experienced person to help you. To have an incorrect saddle on the horse could cause them pain and discomfort and to you as well. That, too, could jeopardize your safety.
Depending on the shape of your horse's back, you may need extra padding (I.e. Riser pad).
Each horse and rider are different. I have had extreme difficulty in the past with finding tack that fits BOTH the horse and me.