It's a lot more complex than you might think but here's a summary:
Radar capability is expressed in several ways but a common way for overall sensitivity is called "loop gain". That is the "two-way" sum of;
1) signal gains (amplifiers to boost output strength and receiver sensitivity plus antenna gain - big antennas are better - to a great extent but there's more in a complex Radar)
and
2) signal losses (inefficiencies like heat, cable and waveguide losses, amplifier noise, and other simple losses such as atmospheric attenuation or background noise level).
Every Radar has different characteristics. The "two-way" part means it's not just the transmitter output power, it's also the receiver sensitivity. Make a big noise and listen for the echo sort of thing. A loud voice is good and hearing is even more important. The "loop gain" gets a bit more complicated in the details but as you can see, it's a constant and depends upon the Radar, not the target.
Now for the hard part - the target. Big target or small? Flat or round? Aluminum or fabric (a hot-air balloon doesn't reflect Radar very well). How far a given Radar can track depends upon the target too, not just the Radar.
In other words, not enough info to answer the question. However, there is no airplane including the Space Shuttle that can fly so high that altitude alone would make it hard to track with a good Radar. For reference, Google "AN/FPQ-14". They're about 30 year old technology and info in the public domain may surprise you.
<ETA - Well, I just picked "AN/FPQ-14" and THEN I Googled - not a lot to see. There should be quite a bit of info also available by looking at "AN/FPQ-6", "AN/TPQ-18", "AN/FPS-16" or "Eastern Range" or "Western Range".