First off, conifers (Evergreens) are sold, buy the foot, like wise the spade is chosen based on it's height, deciduous (hardwood or shade trees) are sold by their Caliper (trunk Diameter) and the spade size is based on that.  I've been to auctions and a buyer is excited the got a 14' maple for such and such amount, the thing is that trees had a good balanced fertilizer program with no competition for nutrients, so yes it's tall, but the caliper isn't that big, it's a young tree, the bigger the caliper, the older it is.

So that all said, an Evergreen 16'-18' tall should have a root ball 50"-55" in dia. now, since you're not selling these you don't care how large of root ball it is or how much dirt you pull from the field, so bigger is better. You could probably get away with a 44" depending on soil and weather conditions, It's up to you. the bigger the ball, the more roots and less stress on the tree.  And it will take some time for it to reestablish itself.  Depending on what you do for a hole, I would suggest staking it depending on what your winds are like and how much rain fall.  Talk to your local tree mover and ask him what they do, then ask what's best :) and then you decide. 

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