The word voluminous* suggests that something has a lot of volume (that is, it has a lot of room inside it) and doesn't make sense in this context considering mirrors are generally very thin. A mirror would likely be referred to as *wide*, *tall* or more simply *huge*/*large*/*big*/*giant . You could also make the first sentence more natural by using the phrases: - "her washroom mirror*" rather than "her mirror *in the washroom ".
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