Terrain size in unreal is exceedingly confusing. The best thing you got is google “unreal terrain technical guide”.
You cannot easily break a terrain into pieces. It is not a regular polygonal mesh. It operates much like the grid does - the further away from it you are, the larger it’s geometry grows, the closer you get, the smaller the geometry gets. This is the key idea behind Level of Detail, aka LOD’s.
But Terrains is kind of an advanced subject to be honest - you have to understand how level of detail works in 3d in general, and then understand what the purpose of a terrain is on a technical level. You don’t have to have advanced knowledge to use terrains to a basic level, but to be able to make them be a desired size and understand and measure performance implications does get into advanced territory.
Will extra terrain drag down performance? You just have to test and see. Depends on one million factors. Hardware, platform, resolution, shaders - nobody can say, you have to test and see. You can use world composition to break a terrain into streamable chunks - but again that is more advanced stuff you probably don’t need to even think about as a beginner.
If are just wanting to get productive making a scene, I’d just drop in a large terrain and go to town. As long as you don’t have the resolution of it turned up stupidly high you won’t notice any slowdown. It could be 100kms wide and probably you wont notice anything unless you have a potato machine.
Get comfortable using unreals placement tools - really they are cream of the crop as far as level designer tools go.
If you are wanting to get serious about realistic natural terrains in unreal, I’d recommend taking a look at world creators guide - they break it down about as artist friendly as you can get. But even that will be a lot to chew on for a beginner. Unfortunately, much of this stuff depends on foundational knowledge you just can’t get overnight.
Fortunately youtube is overflowing with Level Designer/environment artist beginner friendly tutorials. Doesn’t matter if they are great tutorials or not, anybody can get you familiar with the basics of working in unreal, using the placement tools, and so on. A week doing tutorials like that and you’ll be very efficient in blocking out levels.