I haven't personally done a bed in a bag (prefer duvet covers), but DD bought a king or queen BinB set at costco a few months back and was very pleased with it...not sure about the size of it, but usually costco has pretty good quality items, and size is sometimes related to quality. Hopefully the bag will tell the size.

On the overstock.com website, they have many reviews that help with your decision-making, shipping is either very low or free, and they have the measurements listed so you know ahead what you'll be getting. I've linked to their bed in bag listings and they should be sorted by best reviews.

I bought a Croscill duvet cover set from overstock that was generously sized and have really been happy with it. I've found that if you receive something that is not as advertised or pictured (either color, quality, or size), just call customer service, and they'll give you a return free shipping label by email. I have used that twice and each time it was very hassle free. No, I have no connection to overstock. They're having a sale right now, and in addition that link below should give about 10% off, or you can google for their coupon codes.

I also bought an oversized duvet cover from Wayfair.com and found it to be good quality, and their customer service at that time was helpful also.

Another source is the company store. Good quality and usually oversized items. Good luck!

Here is a link that might be useful: Overstock bed in a bag stock