How can I make a sticker be in an rectangle GIF?

The max sticker size is 618 by 618. Most are 408 by 408. The images app is a bit different than the stickers, don't use that as your guideline.

Also another limitation is the 500 kb file size. Getting large size animated gifs under that limit is difficult, especially at a size of 1280 by 720.

The Sticker Pack project templates in Xcode will enable a square grid with fixed sizes. If you want rectangular images then you would need to implement it as an iMessage App Extension and have some custom UI for the sticker pack. If you're just looking for a grid but with rectangular images you could probably do that pretty easy with a collection view where the cells us MSStickerView to hold the stickers.