So I am thinking of using donor fork for a frame and was wondering couple things;

1. If its too short, then (although it sounds obvious) is it not usable?

2. If its too long, do I just get it cut? To match original fork?

Long story short, fork needs steering column repair, will hold off and decided to use a donor fork for now. Heard its not that big a deal...other than the mis-matching frame and fork.

Threaded headset or threadless? Steer tube diameter must match the original. Too short will be unusable in any case. Too long, and you can generally use spacers. If threaded headset, the threaded section must extend far enough to hold the headset parts, otherwise you may need to extend the threads (usually a bike shop job, as the tools are fairly expensive for a one-off job).

Handling may differ from the original fork if the rake is significantly different.

It is 1'inch threaded. Ive been told it shouldnt differ, given its not drastically different. I have a local selling a unicrown fork, ebay has tons, but trying to get a forkg with Campagnolo dropouts..

Ive been looking at Italian forks mostly, and the longest one just so Im 100% sure it will accomodate/match my fork.

The steer tube on the replacement fork should be as long, or only slightly longer than on the original fork. If longer, you can either use spacers to occupy the additional length, or trim to match the original. Thread a steel headset cup down to the point where you want to trim and use it as a guide to keep the saw straight. If you have two threaded cups, even better, as you can thread the first down to where you want to trim and thread the second down just far enough to allow the saw blade through. After trimming, unthreading the first cup will clean up the threads. Use a file or deburring tool to deburr the inside of the steer tube.

And make sure the crown race seat matches that of your headset. You can mill a JIS (27.0mm) crown race seat to accept an ISO (26.4mm) crown race, but not vice-versa.

steel steering columns CAN be "replaced" that is: new length of same diameter spliced to a stub of the original (that stays brazed to the crown). It allows a 'too short' to become longer (just right) and a threaded to become an unthreaded.
As John pointed out, along with the total length being important, the threading must be long enough, too, so that the HS will work, whether spacers are used or not, the threaded upper cup has to screw down far enough to make contact: you can't compensate THAT with spacers.