So 15 man is good because it doesn't have a big effect on you, but it would for 25 mans who would end up disbanded due to the drama of cutting half their raid team (you'd be surprised how many internal cliques etc a healthy 25 man has and how one player stroked against the hairs can start shit) :P?
I like 20 man, tbh. There's a far bigger chance of 10 mans surviving by merging 2x guilds with a similiar mindset, than there would be of 25 mans surviving a cut-down to 15 people. And you'll see far fewer people end up guildless and searching for a new home (10 mans will be merging, 25 mans might have to cut 3-4 players vs 25 mans having to cut half their roster).