So I’m trying to work out a large enough map that won’t choke peoples pc’s with a flying game that will have up to 8v8 multiplayer. Games like BF2 and Star Citizen have some huge maps. Should I do level streaming? I just want the maps be about 5 miles on the 6 and y and few miles up on the z. Any suggestions?
Its hard to give you a specific number. I would create a prototype map to get an idea of the size. As far as optimization, use Level Streaming and World composition. Make sure the “map resolution” is not too high (think of the flying simulators and how they have huge maps but not very detailed maps if you get close).
I would highly recommend you check out the training livestreams for Getting Started with Landscapes and the follow up to learn about landscape optimization.
Good luck! 
Thanks for the input. I was considering level streaming, I just need to verify if you can see a moving enemy from x distance if they are on the other side.
You’re going to need to do a whole lot of optimization. LODs and fades in materials are a must. When I say fades in materials, I mean things like fading normal maps to flat or killing off complex clusters of nodes with simpler ones; past a certain distance.
Another big thing you’ll need to work out is maximum flying speed. This is inversely proportional to the amount of content you need to stream in. The more complex the world, the slower you’ll want the max speed. Otherwise, you can run into issues where assets don’t load in fast enough. In other words, you’ll really need to have a hardware target in mind.