The Fusion 360 approach to parametric-driven designs has some issues that apparently OnShape has figured out. Ryan says parameter changes in OnShape does not break things nearly as often as in Fusion.
Sometimes it seems the breaking is less likely when the parameter changes are smaller amounts of change.
I made several different sizes available, and one of them may do better on the edit to your desired size, if the starting point is closer to your end goal. However, they are apparently all for wood rails tables instead of superstrut metal rails.
If I can get some time, I can edit and fix one at the size you are aiming for. I cannot get to it right away.