As far as I'm aware, using Calibre to create your EPUB should automatically sort out any necessary image scaling so nothing runs off the screen. You shouldn't need to edit anything manually.
If the images are inside the book (i.e. not the cover) make sure that Convert - PageSetup - Output file is set to one of the Sony options. I use 'Sony Reader' for both my 6" PRS650 and 5" PocketBook 360. I think it uses something like 590x775 as the absolute max size, but it will reduce that accordingly, depending on the PageSetup margins you've set. It's always worked for me.
If it's the cover you're having problems with check out the cover options on the Convert - EPUB Output page.
The above options are per-book. If you want to set them as standard defaults for all books you can choose the same options in Preferences - Common Options - PageSetup and Preferences - Output Options - EPUB Output