If your TCad version offers "Constraints," one approach is to Constrain the outline of the plate and the outlines of the internal features; then apply constrained locating dimensions to each of the internal features, and apply constrained size dimensions to the plate and to the internal features. In the Formula column of the Calculator Palette (where these dimensions will appear as Variables), express every locating dimension, and one dimension of the rectangular plate, as a function of the other dimension of the plate. Then, whenever you change the aforementioned "other dimension" in the Calculator Palette, both the overall size of the plate and the locations of the internal features will automatically change proportionately.
The attached screenshot shows the Calculator Palette entries for a simplified example of your problem: a rectangle containing one small internal rectangle and a small internal circle. "Var1" is the width of the rectangle; "Var2" is its height; "Var3" through "Var6" are the locations of the internal rectangle and internal circle; "Var7" & "Var8" are the dimensions of the internal rectangle; "Var9" is the diameter of the circle. Changing "Var1" in the Calculator Palette changes "Var2" through "Var6" in accordance with the formulas and leaves the others unchanged.
If your version does offer "Constraints" and you're not familiar with the concept, you'll probably want to consult the Help file on this topic.
*EDIT: Maybe I'm reading more into your question than I should. The attached "Rectangle with Internal Features.gif" shows the setup I referred to above, with three different values of "Var1." Note how the sizes and the relative locations of the internal features remain constant.
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