You can put areas for Players to draw or type things, if you want them to, for example a Poll ("Which adventure do you want to play next? Type your name under the title"). Reasons to make a much larger page could include ideas like: Displaying the World Map or multiple maps in different scale (the town, the continent, the world) spreading a bunch of illustrations a place to keep a Portrait of every NPC that the party has met in the gameplay GM can put a bunch of pictures/illustrations in the margins, in the background, perhaps hidden under Fog Of War or blocked out by Dynamic Lighting walls, and then the GM can use "Shift-Z" technique to pop-up the pictures, perhaps making a sort of introduction.
For that, anything like 8x11, 25x25, 25x30, is enough.
(2) A place to park PC tokens, during set-up and for purposes of organizing/collecting the party, marching order, and let the Players Ribbon stay there so Players can access their Token in-between game sessions. I would say the usual 2 reasons for a Splash Page are: (1) A cool picture, logo, or scene that's the first thing that Players see and which avoids starting the Players on the initial Map/Page of the gameplay. Something neat is, the GM can always expand the page size later, if they have a reason. Do you want any special utility / purpose for the Splash Page besides the usual? There can be reasons for making a larger splash page, if the GM wants some extra functionality there. A wide variety of sizes can work fine for a Splash Page.