Selfie camera needs a low resolution, full sensor mode
Sebastian says that 640x480 gets there by cropping: https://source.puri.sm/Librem5/linux-next/-/issues/315#note_156311
Half-size (preumably full sensor) mode is 1632×1224. That means the quarter-size mode would be 816×612px.
Having this resolution would be useful for photo camera purposes: the photo is most likely going to be taken using the entire sensor area, so a preview should offer exactly that as well, at a lower resolution to preserve power.