Dear 28 Days,
Not sure on how much SUP experience you have, but;
Board Thickness;
Obviously if your board is 5"3/4 (+grip) or 6" thick, you'll need to match it with a longer paddle, as opposed to the Starboard Race Canoe's & Foot Spa's (

), with "dug out / recessed" foot wells, that has you standing maybe at the effective water line.
Width;
Narrower boards are generally less stable, and sometimes faster. I said sometimes.