PD of PID in a balancing robot

I just made a measurement:
height: 28.5cm
width: 30cm (outside of one wheel, to the outside of the other one measured)

By my cnc machine and good software, I can easily change construction, to hit the goal I'm ready to reconstruct it as many times as needed. Could you please tell me more suggestion and I do a complete reconstruction?

May be you know a better model, or a proportionality of sizes that helps in addition to increasing the moment of inertia, or may be there is some numeric proportionality already known for such models of IPR?

I can make design close to specification if I have one, in inventor software, then cut it with cnc so to at least be more sure about the chassis

I also draw simply what I understood from your post, you know I'm not native in English and not even very good in it, I sometimes can't get the right imagination from reading the text..