Perhaps it's better not to use pixel graphics but vector graphics.
Link: What's a Vector Graphic?
Vector Graphics, unlike Pixel Graphics, scale very well. For example when someone prints a large poster or billboard they usually design their graphic using a vectoring program.
Inkscape is free and can do the job.
If you have access to the Adobe software package you can use Adobe Illustrator. When you know how to use Photoshop then you should feel right at home when using Illustrator.
Why am I recommending this?
At these sizes memory becomes a huge issue. Of course one can buy tons of RAM but this is pretty expensive when one considers, what's the goal.
Usually one wants plenty pixel because they want to print very big or with very high DPI-Amounts.
Using Vector-Graphics solves your Memory-Problem as well as the Scaling Issue. Vector Graphics store mathematical construction data instead of rasterized color data.
Thus a vector shape can be re-rendered for any arbitrary scale - because the image is re-constructed using the mathematical description of the image instead of the pixel values.
Common vector formats include SVG and EPS.
There are lots of tutorials out there for peopIe who want to juml from pixels to vectors. It's actually quite easy because Illustrator does the heavy lifting for you.
hope this helps.