This poem has two distinct settings. In the first stanza, the setting is probably a bathroom, because the wall is speckled pink, and there are a lot of faces and darkness in the room. If the room were bigger, we're guessing that the opposite wall would be a little more interesting, and the lights wouldn't be off so much. But the mirror could be anywhere – a bedroom, a hallway? It's left up to our imagination. We do know that it's a four-cornered mirror, and not an oval one, so this mirror is probably just practical, and not super fancy. Also, the detail that the wall is pink might hint that there are women in the house, and that the mirror is in an area they use. We imagine this mirror in a bathroom, with perfume and hair gel right underneath it. Where do you picture the mirror hanging?
In the second stanza, the setting changes. We know that our speaker is now a lake, but we don't know how big it is, or if it's in a field, by a forest, or even in the middle of a suburb. We're guessing it's a small, clear lake, somewhere mystical and moonlit, but close enough to a house that the woman who looks at her reflection can come and go easily and often.