So I know you can use PHP: use pocketmine\utils\TextFormat for Changing the color of text but how would I change the size of the text.
TextFormat::BOLD would be how you call it correctly. https://github.com/pmmp/PocketMine-MP/blob/stable/src/pocketmine/utils/TextFormat.php#L71
But if you want to completely change the size to small in Mojangles font, you want to put a special character that makes the text small. Website I use to find special symbols is https://coolsymbol.com/