Yes and yes. Comparing to a tennis court communicates the scale, not the size, which is more important to the vast majority of readers. It's a big thing, close

Yes and yes. Comparing to a tennis court communicates the scale, not the size, which is more important to the vast majority of readers. It's a big thing, close to very big, but smaller than huge. So for a mirror, it's gigantic. Does it really make a difference if it's 45x45 or 53x50? If you're trying to imagine roughly what it looks like, no. Their point is that the thing is close to something you could reasonably sort of picture. I couldn't be less interested in tennis, but I've seen what they're describing at my local park.