The metric system makes it easy to calculate lift created by volume as one kg is equal to the weight of one litre of water by definition (noting salt water has more density therefore the same board will allow more flotation in salt water than in freshwater).
Paddle board dimensions are quoted in feet and inches but volume is always calculated in metric litres. One litre of volume displaces one litre of water and therefore creates one kg of lift, so one litre of board volume will float normally one kg of weight.
For beginners, we recommend body weight in kg = litres x 2 (approximately). Example 80kg of persons weight will require around 160 litres of board for the weight. This means that when standing on the board, it will be pushed about half way under water to displace enough water to float body weight plus equipment weight. Just because a board has more volume does not mean it will be more stable. Side to side stability is a result of the width of the board and the thickness of the rails and to a lesser degree the length of the board. A thicker board can actually be less stable as the feet are then higher from the water and less centre of gravity position. Hence we use this guide for reference.