How can I make it possible to make my game's map very large without it lagging?

Well, this isn't directly a scripting question. You could either use a map-sizer script that can be found in the Free Models, or scale it yourself. Getting rid of lag, however is a different issue. Lag is mainly due to excess Parts in the Workspace, or unnecessary scripts. I'd recommend adding a hat/tool cleaner that would clean hats/tools off of the BasePlate. I'd also go through my scripts to make sure that they're not doing anything major. If they are, I'd try to find a way to accomplish the task, but in a less-complex way, which usually results in less lag (for me at least).