It depends on the circuit you use. I would guess that a signal of less than 10Hz could use its period as a measurement. But since your messing with the pulse width, just stick to a frequency measurement.
The transistors in either circuit can be any handy NPN, like a 2n3904 or 2n2222. The resistor and capacitor values, R & C, are determined as I mentioned before, by t=RCln2.
There are a zillion tutorials for the 555 timer on the web. Theyre VERY easy to use. Configure it as a "monostable multivibrator" sometimes called a "one-shot." The 555 is probably a more stable circuit for doing what you want to do (whatever that is!), but I would still use a microcontroller, and take care of all your needs in one tidy package. It takes a little time to learn how to use them, but you'll open a whole new world of possibilities once you do.