The less impersonal (generic, abstract) the sentence is, the more you tend to use plural for politeness.
There is no impersonal pronoun in Russian; impersonality is conveyed with the sentence structure. This is more flexible, and there are three types of such constructs with a common attribute of pronoun missing.
Indefinite-personal is used when the action is carried out by specific person(s), but this is irrelevant. В магазине мне продали водки. (You don't need to mention salespeople did.) Note 3rd personal plural is used for this type of construct.
Generic-personal is more abstract and is used to refer to generalized cases like proverbs. С утра выпил - весь день свободен. (A "wisdom" that suits any alcoholic.) This one goes with 2nd person singular or, occasionally, 3rd person plural.
Impersonal specifies an action or a state without any definite actor. Бутылку водки занесло снегом. (Abstract natural force did this.) With verbs, it is 3rd person singular.
So, strictly, with the pronoun omitted, the only impersonal/generic construct you can get with 2nd person requires singular, and the use must be abstract as per above.
Colloquially, though, 2nd p. sg. use is not limited to abstract cases. For example, it is very common with directions (идешь квартал прямо, поворачиваешь направо, там парикмахерская). I'd say it's used this way to avoid an imperative that may sound too formal or impolite. Since such use is only marginally impersonal (the directions would work for anyone, but they are given in a particular case), you further go with 2nd p. pl. for politeness.
Finally, there is a colloquial use that further mixes up impersonal and personal by actually getting the pronoun (sg. or even pl.) back into sentence.