300 voters so far, need around 200 more before I can say it's a decent sample.
As for preferable size...people should do what they feel like, if they can. It's a bit sad to see that around 1/3 of 10 man raiders would like to be in 25 man guild but they simply can't atm since current system is drowning 25 man scene slowly but surely.
There's one more thing that people often miss...every time 25 man guild dies a community that said 25 man guild was dies as well. Large, breathing, live community dies. It's hurting the game so much. And with all due respect 10 man guilds are rarely community on its own. 25 man guilds are always one. And having big guilds dying is destroying one of the pillars that made this game great.