The larger the board, the more stable it is. Use your own weight plus the weight of the luggage you will carry with you.
The vast majority of boards can handle a weight between 90 and 140 kg. If you weigh more than 90 kg, you should choose a SUP that is at least 300 centimetres long and 80 centimetres wide, with a thickness of 15 centimetres. This will give you the best possible stability.