I agree with everyone, that you can never give piggies too much space. But here's a suggestion...
If your piggie is really scared, give him LOTS of hidey spots, try boxes, towel tents, hidey huts, pigloos, just lots of places where he can feel safe. Then as he begins exploring the cage more, take them out one at a time, until there are only one or two. It might help boost his little piggie confidence!
Also, piggies are much braver with a buddy, they very much like the buddy system! My Sukki is very adventerous. We just added a second floor loft, and as soon as I put them in, she ran right up the ramp and started doing the happy piggie popcorn dance on her second floor loft, the others came in fast pursuit to see why she was so happy, now they zoom up and down, just for fun!
I think you have mentioned getting a buddy, its a wonderful idea and a perfect solution to your shy piggie. 3x5 is large! Maybe even go out on a limb and get 2 buddies! :-D I have 4 piggies, hehe!