How would you figure out the length of the tension pieces. The window of length would seem very tight. If you make it to short, the boomilever would sit at a downward angle from the loading block angle and if it was to long, the boomilever would also sit at an upward angle from the loading block.

This will depend on your unique design... How are you attaching at the top? How thick is your mounting block? Will the tension rod extend all the way through it? Where will your tension rod intersect the compression beam? Will it extend all the way past the compression beam?

I agree with Retired... Come up with your design, then draw a scale model, either pencil & paper, or CAD... Then just measure.

I can tell you this much: In 99% of the designs, your tension rods can come from 24" sticks and your compression beams can come from half of a 36" stick for ordering purposes.