I just want to add a bit more here as well. I don’t think anyone mentioned this above, sorry if I’m wrong, but one key factor that goes with your lightmap resolution is how well your UV is laid out.
Having a good layout in your UV to maximize space is key to keeping your lightmap resolution lower.
ie. if I were to model a large object and just used something like Flat Mapping in max to throw down all the polys spread out without consideration, I could keep upping the resolution and still have plenty of issues. However, if I maximized my space and laid everything out well I could probably get away with a 128 or 256 (max) resolution.
Another consideration is targeting a specific resolution before hand for your lightmap. This is more for anything that will have hard edges or being used more for modularity.
(A lot of this info if not all can be found on World of Level Designs Lightmapping tutorials)](http://worldofleveldesign.com/categories/cat_udk.php#lightmapping)
What I mean by choosing the resolution is that you will need to place your UV edges in your modeling software before importing into UE4.
ie. If I want my lightmap resolution to be 128 and no higher I would do the following.
You will need to set up the grid in your UV channel.
To set up the grid properly to account for the resolution you’re targeting you will need to do the following: 1 divided by # of pixels you want.
1 / 128 = .0078125 will be the spacing for your grid lines. (more info at this link specifically)](http://worldofleveldesign.com/categories/udk/udk-lightmaps-02-uv-techniques-and-how-to-create-second-uv-channel-in-maya.php)
This may be a lot to take in but a good lightmap can really help!
If you have any questions feel free to ask!