the auctiva 400x300 pic size is just the initial size the pic will show up as in your listing. i do believe that it is 'set in stone'.. however, the great part is that your buyers can click on the picture and see it as large as you want it to be.
i don't know that people tend to 'prefer to have others size their pics'... it's just that 400x300 is a decent average start size for any auction photo, so that's the template norm. i assume most people have bigger pics than that, you just have to click to see it... for example, i personally tend to resize my pics to 800x600 (they d/led from my camera at 1200x800ish)..... they show up as 400x300 in the auction listing, but there's a link below each pic that says 'click to enlarge' and when your buyer clicks that, they will see your picture at the size you uploaded (unless of course you resize in auctiva's image editor).
hope that helps a lil bit.
