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For seeing galaxies and other faint stuff (like planetary nebulas and "blinking" them with an LPR filter) a 2mm exit pupil is best. That's a 10mm eyepiece. Make sure it's at least a good quality Wide Angle one, though Ultra Wide would be better.
For bright clusters and nebula, a 4 or 5mm exit pupil is best, which corresponds to 20mm or 25mm eyepiece. Which one depends on how bright your viewing site is. The 25mm will brighten the background more than the 20mm.
Personally, I do like a 3mm exit pupil (15mm eyepiece) for areas with lots of faint open clusters - but I seem to be alone in that respect, I've never seen anyone else recommend 3mm exit pupil for that.
Technically, the 6 or 7mm exit pupils (30mm or 35mm eyepieces) give you the widest, brightest fields of view (depending on how big your eye's pupil can actually get). On my 14.5" I even use a 40mm eyepiece which gives a 9mm exit pupil - which I don't mind so long as I stick a broadband filter on to dim the yucky background light.
Good luck with whatever you choose, you'll have fun.
Regards,
Renato
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