Settings should match the project needs.
I believe the biggest hurdle in the question is the issue of disk space. I don't think any project should ever be determined by how much "disk space" it will use. If you don't have much disk space, you may need to improve that. It's a mistake to limit yourself due to some constraint such as disk space. Hard drives are cheap.
This is akin to using acrylics to paint a portrait but you don't have any canvases, so you try and use an index card - you are merely doing yourself, and the work, a disservice.
For my work, I start everything as if it were ultimately destined for commercial printing. That means whatever dimensions are required, RGB (so filters work), and always 300ppi. This ensures I can always convert to CMYK if needed later and retain overall quality.
If nothing is ever commercially printed, then whatever dimensions you want at whatever PPI you want. It really doesn't matter. Although, I'd probably stick to something consistent such as 150PPI.