I have an idea. OMFWTG GASP.
How about you keep the sizes and stuff the same, along with velocity. Making it too realistic would screw up gameplay.
What I propose is to multiply every number for velocity, size, distance, and the like. By what number, I don't know, since I'd have to figure it out. When -everything- is bigger, greater speeds seem the same. The only problem would be the strain on your computer, but I have a solution for that, too.

Keep the true numbers the same, but modify the engine to that the numbers reported to the player(s) are multiplied by this number. This will make it seem realistic(er) without making a great change on the engine. By the way, the adding zero's idea is slightly off because it would screw up the proportions.
Edit: Shouldn't this be in the SCP forum?