How small you can make it depends on how creative you get. Keep in mind that you can mount some of these components sideways or upside down, so the space requirements are really three-dimensional not two. I'm going to go through some of the design considerations with you now, but I realize this is a lot of information at once.
I'd recommend leaving the cRIO right side up, as well as the power distribution board. Make sure you leave enough space in your design to plug in all the connectors, and leave at least 1"
(Correction: that should be 2" 
) of empty air over the Jaguar or Victor fans to allow room for airflow. It's good to be able to see all of the various indicator LEDs from outside your power board/box/shelves if you need to troubleshoot. You must have the radio indicators and rockwell light visible from outside the robot, so you may want to mount those separately from the rest of the electronics. Breakers must be accessible for inspection (which means that the inspector needs to be able to get at them with minimal effort, but does not mean that they have to be visible all the time).
You're going to have to change the battery in between matches, so you want that task to be easy to complete. Make sure you secure your battery so it's strapped in and won't bounce around - there's nothing more depressing than a dead robot sitting next to its battery on the field.
You'll need easy access to the cRIO ports to upload any programming changes you make. Also, you have to keep the robot frame electrically isolated from the electronics and the cRIO case is grounded, so don't mount it on metal.
Measure all the components. Make some sketches (or CAD drawings if you're up to that), with reference to the data and power connectivity diagrams so you know what needs to be close to what and where cables are going to be coming out. You also want to pay close attention to the electrical rules (sections 4.1.7-4.1.9), which specify that each motor must have its own speed controller [R50], among other things.
You generally want to minimize the length of your power wiring to reduce losses there. Try and avoid "banjo strings" (wiring that is tightly stretched) - you want a little bit of slack in the ends to make it easier to plug them in and to keep the wires from breaking or pulling out of the connectors. Label both ends of every wire with where it's supposed to be plugged in and what's on the other end, and your life will be much easier when you have to troubleshoot problems.
If you want some examples of what teams have managed in the past, there are a fair number of threads here from previous years where teams have posted photos of their electrical boards. Do some searching.