Smaller screens do make it easier to get a full-view of the game without having to constantly shift eyes and/or neck. I'm perfectly happy with my 19" monitor, which has an actual 16 inches of screen. Unless you intend to be really far away from your monitor, I wouldn't invest in one much larger than 20 inches.
League of Legends IGN: Ellista Heroes of the Storm IGN: Ellista#1823 Twitch Stream:twitch.tv/ellista
i have been using a really small screen for many years. 1280x1024 been using over 10 years i have no need to have a bigger screen i prefer the small screen any ways. im used to it. and see no reason to change