|MadSci Network: Engineering|
You've hit upon a central question of aerospace engineering. However, in order to answer you question directly, I need to know how high you consider Earth's atmosphere to be. Can this be a staged rocket, like the Saturn 5? or must it be single-stage, like the Delta-Clipper?
Even then, however, I could only direct you to an existing rocket. I think you want to know what the practical limit is. The crux of the answer is that we are still trying to build the smallest rocket to leave Earth.
You can review our progress with the list of launch vehicles at http://www.astronautix.com/lvs/orbindex.htm
You might want to look at the Scout family of rockets. They could get to 500 km with a payload of just 150 kg (330 lb). See
On the other hand, the Atlas rocket was able to put John Glenn in orbit using just a little more than one stage.
Your question also leads me to think that you're interested in
understanding how to build a rocket to get into an orbit around Earth.
Visit these sites, they should give you a feel for what it's all about.
There's even a java program for simulating rocket trajectories http://my.execpc.com/~culp/Launcher/Launcher.html
Finally, I should point out that a similar question was asked at MadSci.org before: 897920771.Ph
Try the links in the MadSci Library for more information on Engineering.