Non-campaign maps are 'unlimited' in size unless you configure them otherwise. You may be playing the campaign/tutorial, or may be confusing the visible map (which will indeed expand/reveal as you get near it) for the entire map.
The Factorio map is unlimited (ish, maxint and all that). However the map matters a lot less in Factorio than it does in Minecraft, as the latter relies a lot more on the preexisting world than the former.
It is quite big. And even that might be an understatement. And I believe there might even be unlimited modes. You probably played a forced small campaign mission if you think otherwise.