There is a 300k limit for ePub files on the Sony 505 and from my limited understanding of it is that you CAN open books that are larger than 300k as a whole but only if the book is broken into 300k chunks within the ePub
You could convert it to lrf or maybe even to a compatible epub using calibre, but if it has DRM you need to strip it before you can convert it.
Look for epub drm in google or maybe the epub forum here in mobileread