What is the largest number you can actually visualize?

By “visualize” I mean that you can imagine that number of things, like that number of sheep in a field, or that number of pieces of paper.

We hear numbers like a 250 Gigabyte hard drive, or that the national debt is a trillion dollars, but I don’t think anyone, certainly not me, can visualize a billion or a trillion.

Seriously, what is the biggest number you can actually “see?”