I cannot build a 'big enough' SSTO space plane. I will build a pretty big one, spend days or weeks tweaking the design until it gets to orbit with a nice payload, perform some quick and dirty space tricks, and then spend several more days re-loading saves/rebuilding and re-orbiting until I successfully land it at KSC. Then the moment it lands safely and I have a good, well-tested design, I go start something else. Something bigger.
I went through about 15 iterations of this compulsion until I ended up with a 6,000 ton tweakscaled bohemoth that checked all the space plane design boxes, and FAR indicated it should fly, but it would inevitably explode spectacularly just a few seconds into flight. I spent about a month trying to fix the darn thing before I decided the game just wasn't going to let me fly something that ridiculously big to space.
So then I gave up and reset my goals, and thought 'how small of a payload-carrying SSTO can I build?', and I built one, and then I thought 'hmmn, it's just not quite big enough...'
and here we go again.