In the Measurement palette, you  can set a percentage value instead of absolute value in the W and H boxes. For example enter "50%" in the W box to reduce object width to 50%, or "200%" in the H control to double the height. You can also copy the W value into the H one. Insert "25%=" in the W box, and this will force also the H box to the same value. This provides isotropic resizing. The symbol '=' works also for absolute values: insert "150pt=" in the W box to set both width and height to 150pt.