How to pick the correct ISUP requires much of the same information as picking a rigid board option. Stability is directly affected by shape and volume, most of the same criteria applies. Knowing the skill level, height/weight, paddling location, and how often the intended user will get out on the water will be helpful information to gather.
Newer less skilled paddlers are best suited with boards that have a width from 32”-34”. Board stability is affected more by width than length. The wider your base is the more chance for success.
A paddlers height/weight are important in deciding on board dimensions. Paddlers of smaller size can look for a board under 10’ as long as the width is 32”-33”, and larger paddlers (165 lbs+) should look for an option 10’-11’ at a width of 33”-34”.
The location where you will be paddling will also help determine what board dimensions and style best suit your needs. Flat-water lakes and harbors are the best for newer paddlers due to typically being more calm. A Touring shape (pointed nose) would be best for these water conditions. Open Ocean can be smooth and glassy but always seems to have a gentle roll which can sometimes challenge balance. If your local conditions tend to be varied a Traditional Longboard style shape would be a great choice. This being said, make sure the board width you choose is from 32”-34” wide.
If SUP is your new found love and you expect to get on the water often you can opt for a board on the narrower side of the wide scale as your balance skills will progress quickly. The new board is for everyone, then opt for a board that any size paddler can have fun on.
You really can’t go too big for new paddlers!