The system unit is 150mm wide, 530mm deep and 330mm tall. The monitor is a 21 inch black and white affair. It's 500mm tall, 500mm wide and 400mm deep. The mouse is slightly odd by modern standards - it's a sort of squarish block slightly taller and somewhat shorter than a PC mouse. The three buttons are on the front rather than the top face, and the ball is steel rather than rubber or plastic.
Some photos are now available.
The PERQ 3 can turn itself off and on - the circuit diagram for this is quite interesting. The mains power is switched via a relay, so you must have the battery connected to power the switching circuitry. The PERQ 3 has a boot process designed to make you think it's broken - it's rather odd. Then the machine will appear to thrash the disk for ages, after displaying the 'Graphics Microcode v7.2.0' banner. This is just loading the microcode and kernel, and it takes a while.