No problem! The size doesn't matter because it's a vector, you can make it as big as you want and it'll stay crisp and clear

I've never used GIMP, but assuming it handles them similarly to Photoshop then it should be able to open them and resize them properly, just not edit them very well. For that you'd need a vector graphics program like Illustrator - I've heard good things about Inkscape in the past.

At least it's fun though! I'd probably just use that existing image as a reference, so that if you change anything you can always compare to the original to see if it still matches reality