It's not about size, it's about efficiency. What you want is high thrust to weight ration, and high delta v - both of these come from having as small a mass as possible. The less your rocket weighs, the better.
TWR is literally how much power your rocket has compared to house much weight it has. You can get better TWR by having larger engines and smaller mass. TWR mostly matters on the surface, as it determines your liftoff capacity - anything below TWR 1.0 cannot lift itself. In space, TWR roughly measures how fast you can accelerate.
Delta Velocity is "how much total m/s change you can achieve". You increase your Dv by lowering your weight, carrying more fuel, or using more fuel-efficient engines. Once you get to orbit, Dv becomes your biggest concern, as it determines how far you can travel.
For working spaceships, you really want a good TWR on the get-into-orbit stage, and a high Dv on the let's-go-to-another-planet stage. That means Mainsails on the lifting stage - they have INCREDIBLE thrust, but burn a LOT of fuel to do so, giving them poor fuel economy but the raw muscle needed to escape gravity. Once in space, transition to LV-T30's or LV-N's - these have a lower power, but use fuel much more efficiently and will get you further, the LV-N's especially are 'nuclear engines' that get INCREDIBLE distance.
If I were to suggest a simple 3-stage orbiter, it'd be a large orange tank with mainsail, a few solid boosters around it, and a LV-T30 or LV-N with a small tank. REMEMBER that a VERY small craft with a nuclear or ion engine can actually get ridiculous distances!
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Topic: Kerbal Space Program: Now Hiring Optimistic Astronauts for Dangerous Munission (Read 1448756 times)