Paddles board sizing are relative to what you wish to do with said board.
Short boards 8' or under are generally used for children.
Medium boards 9' to probably around 12' are good for calm lakes / rivers and for some fun in the surf.
Long boards, 12' or more are for the more serious journeys, touring or racing. They are faster and track straighter.
Width is also an issue. Wider boards are slower but are more stable, thinner boards are faster but less stable.
SUP Volume and Weight Capacity
A SUP board must work for your size. If the board doesn’t displace the
correct amount of water for your weight, you won’t be supported. Board
volume and weight capacity are two important factors to consider.
Volume: A paddle board's volume, expressed in liters, gives an
indication of the board’s ability to float with weight on it. The
higher the volume, the more weight the board can support.
A short board can have a high volume if it is wide and thick.
Likewise, a long board can have a low volume if it is narrow and thin.
This means that a person weighing 200 lbs. who wants to ride his or
her SUP in surf will look for a short, maneuverable board with a high
Weight capacity: Each paddle board has a rider weight capacity, which
is listed in pounds on the specs tab on REI.com. Knowing weight
capacity is important because if you’re too heavy for a board, it will
ride lower in the water and be difficult to paddle.
When choosing the length of a paddle board, think about where you want
to paddle. Riding waves in the ocean and racing on a calm lake are
very different styles of paddle boarding and require different length
boards for the best performance.
Consider, too, board length in regards to your type of car, home
storage situation and length of walk to the beach or shore (longer
boards are more difficult to carry, especially in windy places).
Short boards (under 9'): Great for surfing. Short boards are more
maneuverable than long boards. Boards designed specifically for kids
are typically around 8' long.
Medium boards (9' to 12'): Ideal for all-around use on calm lakes and
in the surf.
Long boards (12.6' or 14'): Great for racing or touring. Long boards
are faster than short and medium boards and they tend to track
straighter. If you’re interested in racing or touring, you’ll need a
long board to either keep up with the competition or to paddle several
miles to your campsite.
Board width is an important factor in determining stability. SUPs are
made up to 36" wide to accommodate a variety of body types.
Wide boards (about 31" or wider): Wide boards are more stable than
narrow boards, making them easier to stand on. However, they are
slower than narrow boards.
Narrow boards (29" to 30"): Narrow boards are faster than wide boards,
however, they can be less stable.
SUP Core Materials
EPS foam: A foam core wrapped with fiberglass and epoxy is the most
common board type. Carbon fiber and plastic are also used for board
exteriors. Board weight varies based on construction technique.
Hollow core: Some mid-range plastic boards have hollow cores due to
the manufacturing process. Other high-end boards have hollow cores to
save weight and increase performance.
Polyurethane foam: Heavier than EPS foam, polyurethane is used on
Inflatables: Inflatable SUPs feature PVC exteriors with drop-stitch
construction that create an air core. Inflatable boards are very
durable, light to carry and easy to store.
Fins add tracking and stability to a paddle board. Variations include:
Large single fin: This is placed in a finbox and secured with a nut
and screw. The finbox has a channel for the fin to slide back and
3-fin setup: Also called a thruster, this setup promotes straight
tracking on flatwater and offers good control in surf.
Race fins: These straighter, stiffer fins are best for downwind runs
because they help longer boards (up to 14') track easier in large wind
waves or rolling swell.
Fins for inflatable SUPs: Inflatable SUPs feature either flexible
rubber fins attached to the board or detachable semi-rigid fins that
can be removed.
Hopefully this helps.