I find the small sizes easier to work with, and allow me to fit more objects inside of a custom solar system. Plus, KSP has been there for me when I was a (younger) child and it has fueled my love for space and space travel.
If you find the small sizes difficult, you can download my planets and upscale them to a size that you work well with, but do not pass off the planets as your own.
You should try real scaled planets. Not only is it fun and much more of a challenge, but (statistically speaking) it also means that certain crafts that are built for RSS are better than the ones that are built for the Juno System.