I have a new 58cm frame I am planning to install a 1" threaded fork onto (since I already have the threaded headset). This is something new to me, so please excuse me if this seems to be a stupid question. My question is how long should the threaded steerer be? I am shopping on ebay, and am not sure what the minimum length I should be looking for. The frame size is 58cm and the headtube of the frame is 170mm in length. Is the minimum steerer length then 170mm?
Thanks for your help!

You need to know the stack height of your headset too. You cana get one too long and have it cut and rethreaded, but if it is too short you will be SOL and JWF, too.

No, the minimum steerer length must be the length of the headtube plus the "stack height" of the headset you are going to use minus 1 or 2 mm. If it's longer, you can cut it down or add spacers between the top race of the headset and the locknut. You want the steerer's top edge to be a 1 or 2 mm below the top of the locknut when everthing is installed.

The stack height of your headset should be given in the the instructions that came with it.

you will want to watch out for forks with steerer tubes that have really long threaded sections. Some manufacturers will thread most of the steerer tube so that it can be used on many different size bikes. The problem with that is once your fork is installed there should not be threads going down very far below the headset. If you tighten the wedge of a quilled stem on the threaded part of a steerer tube it can ruin it. Sometimes it won't ruin it immediately but the steerer tube can break later, possibly while riding.

Also be careful buying on ebay. All the careful measurements and thoughtful consideration doesn't mean anything if the quack on the other end doesn't have a clue. He has to measure everything the same way.