To change lightmap resolution, search for “change lightmap resolution” in the docs, and find this page:
Unwrapping UVs for Lightmaps | Unreal Engine Documentation.
Also, this page:
(Scroll down to Lighting)
@ClockworkOcean @jwatte Okay I fixed the problem, I wanted to experiment with Light cone so I increased Outer cone angle from 40 to 89, those triangles appeared, When I switched it back to something smaller it reverted back, Can’t believe I thought it was fault of a light angle and had nothing to do with lightmaps so sorry about that 
Point light?
Spot
Could easily be. You can do all sorts of weird stuff with a point light…
That makes sense – there’s a shadow map per light, and the wider the angle, the more area that shadow map must cover. Tighter light angles means better shadow texel density means less shadowing artifacts!
Yeah, Had no idea it could go this wrong
Thanks man Really great info, I gotta remember this from now on, will try to keep light angle as low as possible within workable conditions