When generating Lightmaps Unity obviously places all your unwrapped objects on one plane and then calculates the Lightmap(s).
It’s Trial and Error to guess the right Texel / Lightmap resolution to not waste any space / create an additional texture. This information is updated at the very end.
So (TL;DR): Is it possible to only update the packing of the Lightmap to get a rough idea of resolution / size etc. Before actually baking?
Unfortunately not possible. It’s been requested in the past, but hasn’t yet been prioritized. For now, you can make a Lighting Settings Asset with very cheap settings (like sample counts = 1, no denoising) to quickly iterate on the lightmap layout.
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