Is there a way to get the GUI size setting of player?

Is there a way, to get the player's GUI size?

No, I would believe that to be a client-side setting.

Okay, but when the function Player::getAutoJump() exists, which checks whether a player has the auto jump setting activated, why doesnt exist that then?

I would say this is because auto jump is a movement related function, and movement has to be processed on the PocketMine server itself.
Meanwhile, client GUI is displayed locally, it isn't handled in any way by the server.