1. When using either as a pronoun or an adjective, use it to refer to one of two items. If there are more than two, use any.
Either of the two bachelors is worth dating (good)
Either of the three bachelors is worth dating (bad)
Any one of the three bachelors is worth dating (good)
2. When using it as a conjunction, you CAN use it with two or more elements.
He left his heart in either San Francisco, Portland, Seattle, or Vancouver.
3. When it is a pronoun, either always takes a singular verb.
The two girls enjoy ice cream more than either enjoys ice tea.
Either of the guys has enough money to buy a condo.
4. When the elements of an either…or are the subjects of a sentence and are singular, the verb is singular.
Either Dave or Dick is going to play lead guitar.
5. When the elements of an either…or are the subjects of a sentence and are plural, the verb is plural.
Either the tall guys or the fat guys are going to win.
6. When the sentence mixes singular and plural, there's no consensus among the language pundits. Some believe that the verb should agree with the noun or noun phrase closest to it. Some don't. You be the judge.