Ordered primarily because when we go on holiday, we know the bedroom curtains are very lightweight and we wanted to have something to help block out the sun. As such .. the size is a bit of a guess.This is very lightweight and portable, and blocks the light well. Obviously it does depend on the amount of window it could cover and this is where this is let down just a little bit, as it suggests that, within reason, you can "adjust" the fit: "The lightweight material allows you to customize any shape or size to fit your window.". This is indeed the case, but the fact that there are only a few places to fix the blind using the suction cups mean that it isn't as flexible as I'd like. I have, however, attached photos using a "test window" both with and without the blackout blind up - on a fairly dull day, and with fairly thick curtains, but it is still obvious the blind is blocking out a reasonable amount of light.The suction cups themselves stuck well to the window, although were a bit of a struggle to get them attached through the holes on the blind.Overall, a great idea especially if you're going to be sleeping somewhere other than at home and would like to block out some of that pesky light.