Update: I made a little test, as for KSP 1.2, you can only fit 50 meters wide structures on the VAB.
We would need some VAB mods for at least 80-100 "free" meters in diameter for objects to fit for at LEAST construct the entire 64m Endurance.
Kerbin is about 10 times smaller than Earth, the gas atmosphere goes on about 64km, on Earth that'd be like 500-640km?
The rockets power/thrust are O.K., but we have 10 times less fuel... Air pressure, drag and other stuff are still the same on KSP, I think (Kerbin's density is 10 times higher that Earth's, the atmosphere has the same density, but it's 10 times smaller - BUT reentry speeds are the same, that's quite buggy, it means that: the ships should have the same speed/velocity as they would have on Earth, that's why we need the same thrust power, but only 10% of the fuel... I think ;-; ) There's this basic idea I had: the KSP Endurance should have the same 64 meters size.
The MK3 Cockpit is about 4.2 meters in diameter. That's half similar to Nasa's actual space shuttles, I even have a mod that adds hull/fuselage/rcs stuff to the stock parts (Cormorant Aeronology 1.2). They are HALF the size... In order to fit the MK3 cockpit.
There are other mods that make the KSP solar system FULL SIZE like ours, then the ships are scaled 2 TIMES BIGGER, but the Kerbals don't scale with those ships, they are tinny next to them (close to human sizes maybe?).
THE SHIPS HAVE HALF SIZES on stock KSP and Cormorant Aeronology 1.2, we only need 10 times more fuel and DOUBLE the sizes if we used Real Solar System mods... the thrust powers are the same still, right?
That means: the Endurance SHOULD be 64 meters in diameter. Then the Kerbals should fit nicely on the IVAs and stuff, I don't know yet surely...
ANYWAYS, I just made this chart to make stuff easier: http://imgur.com/kBNnHye
http://imgur.com/XIhVC5y
http://imgur.com/88Cqu4t
Ok, now I'll wait for @JPLRepo to tell me what he wants to do Ç o Ç