If we can't travel outside the galaxy how do we hypothesize about what the galaxy looks like? Or know what dimensions it might be? Or really know anything about the galaxy?
All of the stars you can see in the night sky are stars in our galaxy…we can figure out how far away those stars are from us (using a bunch of very cool and complicated techniques) and that information gives us a pretty exact idea of the size and shape of the Milky Way (and where we are in it!)
Just remember: even if you can’t slay dragons and shoot fireballs from your hands, you can step over small objects in your path, and that makes you more badass than a lot of video game characters.