Upscale? The original was 120” in diameter.
Some notable “Big” Gemini-Titans have used the Revell 1/24 scale Gemini spacecraft with.a 5” tube.
These days, you can probably find a decent 3D STL file of the spacecraft and get it 3D printed for what’re scale you’d like. I think BT-80 (2.6” would be pretty nice.
“Standard” scale sizes, like s 1/35, I do not scale for as it only woks for a very limited number of models. I’d rather go by what approximate size of model, and engine power range, interests me. For example, I have done “sport” shuttle models at 1/110 scale, because its a nice size and matches BT-55 and 3” tubing, And flies nice on D12 power. “Just” taking it to 1/100 scale would not only require custom tubes, but significantly more mass (cube loading), and drag, so it would require an E to fly about as well as a D12.
Also for example, my 1/39.5 scale title Joe-II models. Because a 12” wide plastic sheet with 120 corrugated so, when curled into a tube for the main body,, defined the model scale at 1/39.5.
And actuallly the scale factor usually comes down to what basic part can be used such as a tube diameter. Or a unique part like plastic mode spacecraft, or a certain model rocket nose cone shape instead of having to fabricate a scale nose.