Can someone help understand what these different RF power detector specs mean?
Here is an extract from the LT5534's data sheet. Note the part enclosed in a red box: -



What that line is saying is that the input power range is -63dBm to -2dBm i.e. a range of 61dB as indicated in the line below.

If you are still confused then 0(zero)dBm is 1 milli watt and if the circuit impedance is 50 ohm this means: -

\$\sqrt{P R}\$ volts or \$\sqrt{1\times 10^{-3}\times 50}\$ volts = 223.6mV RMS.

If you are still confused -63dBm is a power level of 501.2 pico watts and -2dBm is a power of 631 micro watts. I think you should be able to convert these to voltages by now.