I've ran 25, 28, 32 and 42c on my mavic 19mm. All with equal degree of success in terms of "mounting"
I'd say don't go less than 28mm. - Not for mounting reasons, but for sheer comfort reasons. There's literally no speed penalty for going bigger than 25-28. I used a 32c tire over those two. - I tested and concluded if you spend good on your 32c tires you'd get same/better speed performance of a smaller tire size. - Oh and the mountains of comfort that follows.
Though, I recently switched to 42mm tires which are about 800 grams heavier than my 32c tires - (400 per wheel) but the weight penalty from 25-28c was only 150 grams per wheel. Sadly, it's hard to find tires past 35c that aren't orientated at Puncture resistance or Off-roading, so they're generally heavier.
That said, I'd recommend these Heavily
http://www.chainreactioncycles.com/vittoria-zaffiro-pro-road-bike-tyre/rp-prod109595 < select the 32c option
On sale right now and they're insane value for money given the weight (<290g) puncture resistance, grip and durability.