Board Specs (Terms):

Concave
The rail rocker stays essentially the same yet the middle stringer area of the board bottom is carved out to "reduce" rocker and give water a faster transit time. Concaves also give the board "lift" (as do harder edges) and seemingly pick the board up out of the water creating more speed or drive.
Tri-Concave
Features a single concave in the middle of the board that splits on either side of the stringer generally between the two leading fins into two separate concaves that flow into a flat at or behind the third trailing fin.
Single-Concave
Single Barrel concave running down the middle of the board.
Double-Concave
Two deep 3/8" x 18" x 5" elliptical concaves on either side of the stringer usually before and in front of the leading two fins.
Deep-Dish Concave
Generally a large pizza attached to the bottom of a board to quickly hand to the main enforcer out at first peak... a pronounced single elliptical concave in the mid to back area of the board creating a feeling of ball bearing or flying bubble effect for insane speed in all conditions. This was my secret weapon for making super fast, fun boards during my tenure at Quiet Flight. My boss used to yell at me not to make them and I still did anyway because I knew they worked! Now some form of concave is the mainstay of all modern surfboard design.
Size and Width
Shortboards generally are 2" taller than your height, add length and width for stockier builds. Add 2" to 3" length for every 10 years over 20. Add 6" +/- length if surfing for less than 1 year and then reduce the size of the board as conditioning and experience increase/improve, rather than the reverse which is typical these days. Generally more "volume" equals more fun especially when learning.
Volume
The actual amount of foam which creates "flotation", which is somewhat like measuring a shipping box:
length x width x thickness = volume.
Rocker
The dimensional curve of the bottom (non-waxed side) of the board.
Length
The length is measured on the bottom of the board from tip to longitudinal tip (nose to tail).
Width
The widest point of the board. Generally at the center or 2" behind center of the longitudinal "length" on modern shortboards.
Thickness
Calipers are used to measure the actual stringer thickness. Generally above the center of the board.
Glassing
(Strong) 6+6+6 is two layers of 6 oz. fiberglass high quality cloth on the deck (where you abuse the board) and one 6 oz. layer on the bottom. The weave tightness and fibril density, or "size", determines the cloth weight and thus its strength - Yes, weight = strength!
(Regular) 6+4+4 is lighter, but also more likely to get damaged.
Pro" or "light" (4+4+4) is even lighter yet, but also shortens the life of the board considerably.
Super (foolish) Pro, 4oz deck and 4oz. bottom.

It's your call, so choose the combo that best fits your imagination!