Most compact high voltage source for making long arcs
You will need 3 × 10^6 V (three megavolts) to achieve a spark in dry air. That's rather a lot.

However, people have built hand-held Tesla coils that achieve roughly what you're asking for: http://www.rmcybernetics.com/projects/DIY_Devices/plasma-gun-2.htm (and they'll even sell you the parts!) As "Guess Who" points out, it's quite a challenge to avoid insulation breakdown inside your device.

This exploits the tendency of charged sharp points to spray ions into the air, reducing the breakdown voltage considerably.