It should probably be pointed out that today you can already set the size precisely using the set width and height dialog (under the resize button in the toolbar), which I find easy enough.
A valid point, although personally I don't really use decorations much on the BD, and when I do, I don't need precise sizes that badly.
For the moment, at least, you could probably write a QD shortcut which will pop up a dialog. There might even be one already in the group. You could probably even call the NI dialog (in <LabVIEW>\resource\dialog\resize.llb). I think it simply uses the selection list property and it has a boolean panel input, which I assume would take the selection from the BD if it's false.
Not an exact solution but my property browser tool can help with this. Just select the decoration and find the size, double-click the number, and edit. For the block diagram decoration be sure to turn on the "Scripting" filter.