Panasonic Lumix DC-S1 Owner's Manual page 84

5. Image Recording
[Picture Size] is not available when the following functions are being used:
0
– [6K/4K PHOTO]/[Post-Focus]
– [RAW] ([Picture Quality])
– [High Resolution Mode]
– [Multiple Exposure]
You can register functions to Fn buttons:
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[ ] [
(l 279)
84
] [Fn Button Set] [Setting in REC mode] [Picture Size]