Modern sprint kayakers and surfskiers all use the more efficient “wing” style kayak paddle that is characterized by distinct concave scoops in the power sides of the blades and a non-symmetric oval in the blade shape where the “top” or inside edges of the blades are longer and more curved than the bottom or outside blade edges. This shape lends itself to greater efficiency and speed - IF you use decent paddling technique.
Wesley Echols' discussion and review of wing paddles covers most of the important features that you should be considering in your choice of a wing paddle. As with most aspects of surfski paddling, there is nothing more valuable than experience to guide your ability to make good purchase choices.
Smaller or less powerful paddlers usually prefer smaller blade sizes. Stronger paddlers may also go with relatively smaller blades if their paddle sessions and races tend toward longer distances.