Combined Mesh + Texture Atlas = Distorted lightmap

In order to decrease the drawcalls of our game I'm trying to combine meshes & textures using "texturePackerStatic" (combines meshes & textures with the same shader). Now when i try to lightmap the combined mesh I get a few problems:

1) The lightmap resolution will be too big so I have to lower the scale in the lightmap. I tried to counter this by increasing the max atlas size. However I still get 512x512 lightmaps (even when i select max size of 4096x4096). 2) When I bake the lightmap I'm left with a completely distorted mess. 3) I tried to "lock atlas" on the lightmapping panel but get the following error: "Lock Atlas is enabled and lightmap indices on objects in the scene don't index into existing lightmap slots. Make sure the indices are correct and that the lightmaps array is big enough."

I'm running out of ideas. :(