Size of Potentiometer

How much current do you need? What are you using that output for? This question is pretty much impossible to answer without more information. (It's underconstrained -- any potentiometer can adjust 9V all the way from 0V to 9V, but using, e.g., a 1 ohm potentiometer will just kill your battery in a few seconds. :-) )

Oh, I see. I'm regulating one led for brightness and eventually a series of them.

Small adjustable standard or low dropout three terminal regulator would work well. With a drop of 2.5 volts per led you can't put many in series (conserves more battery power)

Depends on how long the battery has to last etc..

Adjustable current source may work better or drop less voltage - or current mirrors to work several series strings.

You cannot precicsly control the voltage, but all you need to do is control current. The potentiometer can be series wired, along with a limiting resistor for the max brightness, also in series. You ned to know the current range of the LED, and the voltages needed for series strings. You can only use a small number of series LED's before you run out of voltage. A 2K pot with a 270 ohm resistor would get you going.

greg

Thank you everybody!

This may not be obvious to the OP, but I think what you are saying is to use the pot as a variable resistor, with connections to 2 of the terminals only. Using it as a pot wastes power.

Tam

No more than two inches in diameter...