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.

Large area measurements don't actually come up all that often for most people in real life so if you say something is 1200 m^2 most people won't have a lot to compare it to.

Maybe they remember the area of their house and that will give them some idea (it is around 9 times the area of my house).

Most people will have a decent idea of the size of a tennis court. Their school likely had one that they would have seen when taking physical education, even if they never actually played. They also show up in movies and TV shows.

Same goes for basketball courts and football fields. They are probably actually better than tennis courts for this because basketball and football are more likely to be televised.

We’re not the demographic they’re looking for.