I don't think that writing a FAQ about increasing the maximum memory would be interesting if its purpose wasn't to let users increase the default maximum
You can choose any value for this maximum but don't select a value close to the RAM size of your computer, because it needs some memory for other tasks too. Choosing a large value like 32 GB won't probably increase performances, except if your home design uses many very large 3D models and texture images. You should think about it the opposite way: a too small value may prevent Sweet Home 3D from computing photos or slow down a lot other tasks.
If needed, you may follow how much memory Sweet Home 3D actually uses in Activity Monitor application available in macOS utilities (but the displayed memory is not easy to interpret because Sweet Home 3D is a Java application).
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Emmanuel Puybaret, Sweet Home 3D creator
