What’s the size ratio of 1st stage and 2nd stage?

[Julius03]

I have played KSP for quite a while and now I’m trying to design a reuseable two staged rocket. Kind of like SpaceX’s Starship. I just don’t know how I could determine the size ratio between the two stages to make it as efficient as possible. Should the 1st stage be 4 times bigger than the 2nd stage? 3 times? What I’m really asking is if there’s an easy way of determining this or should I just experiment like always? Maybe adding more engines or having it refueled in orbit more instead?

[bewing]

Rule of thumb is that a stage should have about 3 times the mass of everything above it.

[Streetwind]

If you would like to understand how the rule of the thumb that bewing mentioned came into being, and how to adjust it for your own needs and purposes, I offer you this post about mass fractions for further reading.  

[Fierce Wolf]

I measure in Delta-V rather than size. First stage should have around 3700-4000 Dv to put everything in orbit, and have a little more to return and aerobrake. 2nd stage should have enough to send whatever it is to destination AND brake around the celestial body

[Jestersage]

20 hours ago, Fierce Wolf said:

I measure in Delta-V rather than size. First stage should have around 3700-4000 Dv to put everything in orbit, and have a little more to return and aerobrake. 2nd stage should have enough to send whatever it is to destination AND brake around the celestial body

The reason why delta-V maybe a bad idea is that, without the minimum TRW in a certain altitude, those delta-V are not enough to send you anywhere but down.

[Superfluous J]

14 minutes ago, Jestersage said:

The reason why delta-V maybe a bad idea is that, without the minimum TRW in a certain altitude, those delta-V are not enough to send you anywhere but down.

Same goes for mass, though.

Proper TWR is needed no matter how much mass OR dV you have.