I'd dig wide and deep. You can always cut back what you don't need but I'd get as much root as possible. One of the better ways of removing a plant (speaking from tree transplanting experience) would be to heavily soak the area you are planning on working and then removing the mud by hand. Keep adding water as you go so it can be gently removed from the roots. This will prevent much of the damage done by cutting roots with a spade/shovel. When done it should look like a medusa of roots which will allow for a quick recovery once transplanted. Take some of the soil and gently wrap the roots in burlap or some other oxygen/water permeable material (no plastic).
Since the hop plants are apparently so hardy you probably don't need to go through all this work, but it would allow for a healthier plant that could *possibly* allow for a minor crop harvest (if you don't cut the bines).
Best thing to do if you have the ability is to have the new area for the hop plant already setup. Support in place, soil aerated and compost/fertilized, etc, so that all you have to do is put the plant in. Evenly spread out the roots in a radial pattern so they quickly can get a foot hold and not compete for nutrients.
