I have a somewhat fresh install of Linux Mint (cinnamon) 18.3
Display is @ 1920x1080 and using nvidia 384 (tried 387 too)
I have no idea why the UI is so big but I need to get this fixed because its obviously very unusable.
Could you please try syncing to latest in 4.18 branch or cherry-picking this commit https://github.com/EpicGames/UnrealEngine/commit/706fbd460e8fa569808135d3d93c67fa1e1e4c25 (it will be included in 4.18.2) ?