Output values

Hello :) I plan on messing about with designing my own effects but I have one question.

What values should I watch out for as an output from the pedal? (Voltages, resistances, amperes)? I assume there are values from them even when only supplied by a 9v battery which might damage the amp.

thank you :)

Boss effects (like many others) designed for guitar have 'guitar level' output amplitude, that is -20dBV. Pedals with a little gain are obviously a little higher, but shoot for a maximum output amplitude of .283Vp-p for 'unity gain' (that's close to the -20dBV level) and 2-4Vp-p max for a pedal with some balls behind it. Powered with a 9V battery, you can get closer to 9Vp-p, but that will overload almost anything you connect it to except pro rack gear. Maybe that's what you're after...Pedals typically have low output impedance via an output buffer (transistor or op amp) and the current rating is a function of how your active devices are biased.
LED's usually take more juice to run than many DIY circuits.