dig wide and deep. the tap root will be as deep as the tree is tall. when my parents ahd one that size moved back inteh early 90's, they had a pro come do it. his machine dug a hole about 6 ft wide and deep, but cone shaped so it was only about a foot wide at the bottom. the tree survived beautifully. i have seen people dig 3-4 feet out around a tree and still kill it, others i have seen dig less than a foot out and the tree survives.

take care to do as little damage as possible to the root system and it may make it. you don't have to dig a HUGE hole, but it may well end up being bigger than you expected! and keep in mind that the root ball will be heavy, so have some extra hand around to help move it.

this is really the wrong time of year to move a tree anyway. in my area(just north and east of you) we move them during winter after they go dormant.

I've always read the rule of thumb is to dig 1-foot away from the tree for ever 1-inch diameter measured around the trunk at the base of the tree. Of course I've also read that Spring is a good time to move it since it is starting to grow and will form new roots so it will recover faster. However I've never done this, you may want to actually check out the trees forum, you can probably get much better advice there.