A surface brightness of 25 mag/arcsec2 would be very faint however you may be able to detect an object fainter than this under excellent skies. It depends on whether there are any brighter features to give some contrast against the sky. I have never tried looking at something this faint so I will have to give it a go next time the conditions are right
Regarding aperture have a look at Mel Bartels page on visual astronomy, especially the section - So why then is aperture the dominant factor?
http://www.bbastrodesigns.com/visual.html
or if you want more detail:
http://www.clarkvision.com/articles/...va1/index.html