For batch resizing, get IrfanView. It’s free and the batch conversion/rename tool gives you several different options including new file size and format.
Image size depends on how far you want to zoom in on the original image. You’re in PAL territory so video is 720 x 576. Assuming you shot at the max setting of 3008 x 2000, this means that, keeping the original size, you could easily do a 5X time zoom without any loss.
I just finished a grad video where most of the digitals were 1280 x 960. As long as I didn’t expect to zoom into a head & shoulders shot of one person in a group of 20, I was fine.
Mike