size of wood chips

Again, this depends on the type of smoker you are using. Let us know and I will give you my two cents worth.

I have a 18 smoke vault

I believe that is a propane smoker, correct? If so the size of wood pieces depends on your taste and how often you want to repenish them. I would recommend using small chunks, maybe half a fist sized. They will give you a good smooth smoke for a long time. Smaller chips will smoke quickly and burn up quickly needing frequent replenishing and giving you a heavier smoke. Chips are going to be more likely to over smoke your food and give it an acrid taste.