KrooZa did a useful gargant size chart back in the day. The waaagh seems to be offline so I'm struggling to find it, but a bit of googling might lead you to something more useful than the BoLS thumbnail I found.
Numbers won't be exact for you, but gives a good idea of the difference in scale between the gargant types. Of course, gargants are whatever you want them to be, so long as it's suitably chunky I don't think anyone will be too picky.