Hi Seichi,
Lightmap resolutions are meant to be used in power of 2 (ie. 64, 128, 256, etc). Think of these like you would normal textures you would apply to your mesh. While you can use values other than power of 2 I wouldn’t recommend it.
Also, while you can increase the Lightmap Resolution to get better quality shadows, there needs to be a balance between the value you use and maximizing the space in the UVs 0,1 space. For this reason, thinking modularly or at least in breaking your mesh up into separate pieces that are placed in your level would give you much better results with lower lightmap resolutions.
I would recommend the following documentation as a good starting point for understanding some of the key concepts and working with Unreal Engine.
- Lighting the Environment: Lighting Tools and Plugins in Unreal Engine | Unreal Engine 5.1 Documentation
- Unwrapping UVs for Lightmaps: Understanding Lightmapping in Unreal Engine | Unreal Engine 5.1 Documentation
We also have a Lighting Troubleshooting and Tips guide that we setup on our Wiki to help with some of the more common issues with lighting that people run into.
Feel free to ask any questions if you run into any issues along the way or need help. 
Tim