Edit>Preferences>Background>Size>Custom
I enter 32x32 and click apply, then ok. Every time I close the program it reverts back to 16x16 when I re-open the program.
Purchased through steam if that matters.
Make note of which document you’re applying these settings to, and if it’s for the Active document, make sure you save the document if you want the changed settings to be part of it. If you’re changing the settings for new documents, that won’t affect any existing documents, only newly created documents.
If that’s not the issue: what version of Aseprite are you using? Steam offers a few branches I think. I’m on the 1.3 beta branch on Steam and the background grid works as intended for me.