Oh, yeah, building things to scale can be tricky.
The full version of the sim actually has different unit-systems you can select. So you could pick Astro-Units,for instance, and put your Earth 1 AU away from the Sun.
But even with the free version, and a little math, I did a writeup about the unit conversions that might be helpful for you:
https://testtubegames.com/gsim101/?p=units
My Astro-Units conversion was one of infinite I could pick - but it tends to do a nice job keeping things kinda on screen, moving at a good speed, and also means you don't need to worry about changing the Gravitational Constant at all. So you'd put Earth 2000 m* units away from the Sun, given the conversion. And the Earth's mass would be .002843983 kg*. That kinda thing. Maybe that'll help?
Getting the densities and velocities right will take a little thought - but having the length/time/mass conversion factors could get you started.