I do not know how that could make a big text when sending a message, popup or tip, as the title in MC or whether there is a code to send the title in pocketmine? I'm newbie
At the moment there is no way to send a large popup on the middle of the screen, like you can on for example PC edition, if that is what you meant. MCPE doesn't have that implemented yet as far as I know.
I tried this once and it added a weird symbol everything in add "\n" and also "§l"... That kinda ruins out everything.
Add a §r (pocketmine\utils\TextFormat::RESET) to the end of every line and you won't get weird symbols.