Many current tow boards range from 5’ 10" to 6’ 6" for waves significantly larger than the double over head waves you will be riding. The smaller boards work well in “smaller” surf, although there is substantial debate about the ethics of towing in conditions where paddle surfers are in the lineup …
That said, you should be very comfortable with dimensions such as these:
Length: 6’ 0" to 6’ 4"
Width: 10 1/2 " nose x 17" to 17 3/4" widepoint x 12 3/4" tail
Thickness: 1 3/8" nose x 2 1/4" widepoint x 1 3/8" tail
Foil: Leave a little more volume forward than you would in a short board as it’s imperative that the board has forward momentum at all times - low well foiled rail to keep it in the water at the speed you’ll be going - carry the rail foil all the way to the stringer.
Plan shape: Your choice, although swallows seem very positive with a lot of control over the surface area in the tail.
Rocker: Same as any high performance semi gun.
Bottom contours: Vee or roll in entry, single concave from entry to fins (with max depth under your stance) with double concave or a hint of vee through the fins and out the tail.
Fin placement: Rear fin: 4 1/2" from tail. Rail fins: 13 1/2" from tail, 1 1/8" from rail with minimum toe in, perhaps 1/8" max
Strap placement: You’ll want to use inserts with 4 holes. Back edge of rear foot strap plugs: 7" and 13" and 1" off center of board. Back edge of front foot strap plugs: 36" for one and 40" for the other, each 2 1/2" off center for 5" span between. You can adjust those numbers to suit your stance and the length of the board and you can always add additional plugs if the stance doesn’t feel in sync for you and the board. Remember. You really want to keep the front end of the board down as the forces you’ll encounter when being towed and riding will want to lift the nose of your board.
Weight and glass schedule are very important. Order a 63H for a 6’ 2" or smaller or 68R or 68H for 6’ 3" to 6’ 6". (You won’t get a 6’ 8" board out of either of these blanks.) Foam density: “tow.” Stringer: 1/2" minimum.
Glassing schedule: 4 layers of 6 ounce each side. I laminate 2 layers on the bottom, 2 layers on the deck, then repeat the process - 2 more layers on the bottom and 2 more layers on the deck. This allows me to get a really well saturated and very tight lamination AND lap the rails 4 times.
You’ll want the weight of your finished board to be in the 14 to 18 pound range.
The impact of the board on your “sled” is significant. Hectic, split second, high speed recovery by the driver and surfer puts a lot of stress on the board. This glassing schedule really keeps the board in good shape.