The most important measurement is the steer tube length. If that is too short, you won't be able to use the fork at all since the fork steer tube won't extend past the top of the head tube and there will be nothing to attach the headset to. If the steer tube is too long, you can cut it down or use spacers, but there needs to be threads where you want the headset top nut to be.

Second thing to consider is rake, ie how far the axle is from the line down the frame's head tube (how much the fork bends forward). This affects handling since it affects 'trail' which determines how much the front wheel acts like a caster (the contact patch of the tire is behind the line down the head tube). This is a less sensitive parameter but something to consider if you want to ride no-hands etc.

Finally, there is axle to crown distance, which just measures the distance from the axle to the fork crown. This usually isn't a concern on road bikes since they are all about the same. With mountain bikes or fat-tire bikes or something you need to pay attention to this or your front end will be too high or too low than what the frame was designed for. Once again, this affects handling but on a road bike, you can probably ignore this.

Hopefully someone will chime in if I'm off base and I hope that helps.