I'll begin by saying that you could use a "Glued-up" panel rather than a solid board.
However, if you still wish a solid smaller board, you may remove the "Conforming Vector option on all the lines, arcs, and ovals which have it applied. Make a list of the ones you remove. From there you should be able you change your design-board size and then shrink your carve. Hopefully, nothing gets skewed in the process! If it still will not shrink, look for line constraints on either the fornt or back as needed. Once that is done, you need to re-apply the Conforming Vectors and re save the MPC.
Dave's point is well taken as well.
Rather than removing the Conforming Vectors options, Remove the 12 inch constraint on all the slopes. You will also need to remove the constraints on the outer vector on the Back (on the right edge).
If it still will not shrink, look for other "Line" constraints. Sometimes folks forget to remove these constraints prior to their final save of the MPC. The radius constraints, and centering should not hinder the process.
Each MPC version of this tray, that has been posted has its own set of complications!
AskBud