Generally longer means faster, at least on flat water. But all of the other characteristics below interplay with length to affect speed, and work differently in waves as opposed to flat water. The closer waves are together the more that length typically starts to interfere with turning and agility particularly for flat boats with little or no rocker.
The width at the widest point of a boat. Beam in surfskis occurs close to the seat and surfskis usually do not have abrupt transitions in width so beam can be used for general comparisons of surfski width - generally “narrower is faster” but less stable (flatness of the bottom interplays with this)
The transition shapes from the bottom to the sides of surfski (a hard chine is an abrupt transition or corner, a soft chine is a more gradual curve). Harder chines in a surfski can contribute to quirky or abrupt transitions in handling.
refers to the amount of curvature in the bottom of the hull along its length from bow to stern. More rocker reduces the tendency of a surfski to spear waves so the ends “rock” up. Rocker typically also makes a surfski easier to turn because resistance from the water against turning is more centralized and not extended as strongly to the bow and stern.
The inner space of a surfski that is more or less hollow and provides the overall degree of flotation for the ski. Larger paddlers generally need and prefer larger volume boats although expert paddlers can usually more efficiently paddle smaller surfskis. Larger volume can be preferred in bigger seas and surf. And volume interplays with shape and distribution of the volume in effects on handling. For example - more volume of air in the bow helps keep the bow above water better and also brings the boat back to the surface sooner if the boat does run into a wave.
Differences in weight in surfskis is primarily due to the use of different construction materials. Heavier fiberglass is less expensive. The more carbon that is used, generally the lighter and more expensive the boat. A heavier version of the same model will be more stable but slower to accelerate and slower overall on flatwater (though the added stability may result in greater speed in rougher conditions). Differences in weight may also be important in loading, unloading and carrying your surfski particularly if you have to do this on your own.