Can I use the length extensions and bicep protectors?
On this one, you can use the bicep protectors, but the length extension does not work with this one. The 17" protector is pretty tall. Most people don't need the extra length with the 17".
How do I know what size to order?
Good question... On this model if you are under 5'9" we would recommend the 13". Between 5'10" - 6'3" the 15" is problaby your best bet and over 6'4" the 17" should work. These are general "rules of thumb." If you like a longer chest protector you may want to go up to the next size, and if you like a shorter chest protector you may want to go down. If you have a larger chest & stomach area, you will want to take that into consideration as well.