A 5.125" diameter hole is what it will go through without bulbs in it, if you are putting it straight through. If you angle it, to just get it through to the other side, it will go through a much smaller circle of under 4.5". BTW, it is of very nice quality for plastic. Cheers
Pushing this through a hole rather defeats the purpose. Install a standard light socket, then screw this in to it like a lightbulb. Much easier and more flexible for additional options. I know this post is old, but since people still read the Q&A I thought I'd post another answer.