I allowed for foam. the iMac is 2" tall, it's resting on its side in the box. The box is 4". That's an inch of foam on two sides side, about .75" on the opposite two. I took into account the dimensions of the main items.
The mouse, cables, manuals and disks could sit behind the keyboard (which is only an inch tall, and it would be standing on its end as well).
I just made it green because a) I was thinking how nice the shuffle packaging looked and b) like you said, the target audience and the type of computer it is might lend itself to a bright, attractive and "fun" box.
It's neither here nor there, of course...I just had Illustrator open a while ago and felt "doodley", and wanted to see what a box
could look like if Apple ever started to include a keyboard and mouse with the mini (which I don't think they will, for the record).