Length: Rule of thumb - your Snowboard should reach the height of your chin (rule best suited for all-mountain snowboards). Choose a slightly longer board if you are a heavy rider and visa versa.
The perfect length for you depend on a few more factors. Ask yourself: what type of riding are you aiming for? Choose a shorter board if you ride snowparks. Choose a slightly longer board if you fide off-piste and freeride.
Width: Wider boards are good for lift and helpfull in offpiste and freeride. Narrower boards are faster and more suited for the piste.
Tip: If your shoe size is use greater than US 10 (43 EU) choose a wide snowboard. This way your bindings and boots will not extend beyond the edges and you'll prevent them from griping the snow.