iMessage Sticker Pack Icon Creation

I’m using Sketch 40 for my graphics and am trying to create icons for an iMessage sticker pack.

It would be a simple process if it weren’t for the fact that only 1 of them is equal sided (29 x 29) and so the Scale feature in Sketch is of no use in creating the other sizes that Xcode requires. I have tried using Preview, but even starting with a huge 2000 x 2000 pixel icon the resultant adjusted image is a very low quality resolution.

I am lost for suggestion on how to accomplish creating the icon without having to spend a lot of money. I am a retired person on a limited income.