Hello, @David_Burger 
Welcome to the community! Congratulations on your new iPaint!
I don’t think there’s a way to adjust just the numbers, as it is most definitely determined by the size of your design, as you mentioned. So, I think the only way to make them bigger would be to make the design bigger.
Maybe you can make that suggestion in this post here 
IDEA LAB Vol.07 | Help Us Improve iPaint with Your Real Experience
Here’s a tip: Maybe upload the numbered image into Microsoft Paint, as a guide, and use the Magnifying Glass as instructed in this video.