With an external strong back you have some leeway. I have used the one in the picture for twenty years or more.

As you can see, the uprights to which the forms are attached can be as long as you like. I like mine a little longer so that I can reach under the forms to get to the inside of the hull while I am working on it for wet glue clean out etc.

I also use the strong back for cutting and glueing plywood scarfs etc, by screwing two ft wide sheets of board to the top of it and using it as a very long workbench. For this kind of thing it is nice if it is not too high. In the next picture I am glueing up some scarfed hull panels. I use old car batteries as weights for glueing.

I made my strong back the optimum height for hand planing a long plank and it worked out well for me. Whatever you do, you can always lengthen the legs or cut off a little if they are not ideal.

Et