Hi Jen
The dimensions (especially the length) will depend to some extent how high your crane can lift and how low it can go over the side.
The reason it that you’ll want to be able to hook onto the crane hook from the water (probably by standing in the centre of the boat) and then lift the boat over the side rails onto the trolley.
If the straps are too short you might find the load unstable and you might not be able to lower the boat to the water. If too long, you’ll may find it difficult to hook on afterwards and to clear the rail to get the boat on board or launch it.
Hope this helps
Paul #325 Sydney