After setting them on the grid-line, you can change the maximum width in properties, then unpin the columns and adjust them to your desired space.
I'd say the approach would depend on how many instances are placed and comfortable you are with the family editor and parametric constraints. You can do as the previous poster suggested, and unpin portions of the truss and move them as needed after placement if there are only a few to modify and you aren't going to be making many changes to them. If you only have a few lengths to create, you can draw each length profile in a new truss family and not get too much into the constraints to make it flex if you aren't going to change the length or height at all after placement. Otherwise, you can create a family that will flex with different spacing, but it will take some understanding of building parametric families and figuring out how to set the spacing with various formulas. The best way for us to help for this would probably be to post some sort of work in progress for comments and guidance.