Infinity is fun, and the deeper you get into mathematics, the more fun it gets. There are concepts of different "sizes" of infinity, different "types" of infinity, and all sorts of other weird things.
Here's another thing that fits in your framework that you didn't mention: draw a line on a sphere that meets itself on the other side, like the equator of the Earth. Start walking on that line. It has no start, no end, and if you walk on it at a constant speed, you'll be walking for an infinite amount of time. You'll never reach the end. Does that imply that the line is infinitely long? Does it imply that the sphere is infinitely large? Clearly not the latter.
If you abstract this to one higher dimension, you could imagine that straight lines in the universe "wrap back on themselves" eventually as well. Just because you could travel in a straight line forever and not hit a wall does not imply that the universe is infinitely-large.
For the past 8 months I've been thinking about writing a series on infinity. Maybe now I'll have some motivation to make it happen...
Nice reply. I like the sphere example. That is another example of an infinite system that is not difficult to simulate. It was being told I couldn't deal with infinity in a simulation by someone and then them leaving before I could explain how that inspired this recent rash of posts. :)