The board finder gives you a range from your level, weight and height. This page explains what lies behind it.
Volume
Volume, in litres, is how much float the board has. More volume paddles faster and catches waves earlier but is harder to turn and to push under a wave. A rough start: body weight in kg × a factor that goes down with skill.
The published factors do not agree. For a beginner on a hard board they go from 0.5 to 1.0 L per kg depending on the site; for intermediates from 0.38 to 0.75; for advanced surfers from 0.32 to 0.6. Part of the reason is what is being measured: a foam surf-school board has far more volume than a performance shortboard of the same length. Part is that several of the sources are shops with something to sell. So the finder gives a range, and a few adjustments that every source mentions: age, fitness, how often you surf, the size of the waves, what you want.
Too little volume is the usual mistake. It costs waves, and a surfer who catches fewer waves improves slower.
Epoxy and EPS boards are lighter and float better than classic polyurethane ones, so sources suggest 2 or 3 litres less for the same surfer.
Longboards are sized by length more than by litres.
Length, width and thickness
Length is mostly about wave catching and stability. Beginners go well over their own height, advanced surfers about equal to it.
Width adds stability and paddling speed. Wide is easy; narrow turns more quickly.
Thickness is where the float is. A thick board paddles easily; a thin one is easier to duck dive and feels more responsive.
Rocker is the curve of the board from nose to tail. A flat rocker is fast and catches waves early, a curved one fits steeper waves.
Rails are the edges. Full, round rails are forgiving; thin, sharp ones hold in a turn.
Tails and fins
Squash and round tails are all-rounders. Pin tails hold in bigger, steeper waves. Swallow tails pivot and add float for small waves.
Thruster (three fins): the standard, drives and holds. Quad (four): fast and loose, great in small waves. Twin: lively and skatey, mostly on fishes. Single or 2+1: the log and the smoother, flowing ride.
A simple path
A soft-top 8'0 to 9'0 while you learn to stand and ride straight.
A mini-mal or a hybrid when you ride the wave along and want to turn.
A shortboard when you surf in most conditions and miss turning more than you miss waves.
A fish or a small-wave board for the small days; a step-up if you chase bigger ones.
Try before you buy. A demo board or a week with a rental at the surf school tells you more than any table.