There are mulitiple factors that go into choosing what deck size is right for you.
Style:
If you skate transition (vert, parks, pools) you'll want more stability and surface area under your feet. Therefore, you'll want to skate a deck that is slightly wider. However if you like to do flip tricks, prefer a lighter weight setup, and skate mostly street freestyle (we call this "street skating"), you may want a narrower board. Here are some examples:
7.00" to 7.50": Narrow decks ideal for youth riders.
7.50" to 8.00": Medium deck width for teen or adult riders skating street terrain or doing more technical tricks.
8.00" to 8.50": Ideal for various street and transition terrain. Parks, pools, rails, stairs.
8.50" and up: Wider decks great for transition skating, bigger street tricks, pools, or just cruising.
Shoe Size:
You must ensure your shoe fits on the deck properly. Turning, ollieing, and doing technical tricks requires a good fit between foot and the board. Selecting a deck that is not too wide for your feet and not too narrow is important.
If you were to place your foot perpendicular to the deck, directly center on the deck, you'd want to have just a slight amount of your heel and toe hanging off each end. If you stand on the deck and the deck is wider than your foot length than your deck is too wide. If your whole foot is hanging off by several inches on each end, then the deck is too narrow.