I got mine from Costco for something around £50. I love it, and whilst it would take a general anaesthetic to get me to sleep most nights, I do feel relaxed and rested lying under it.
You need one each if your partner wants one - they are not recommended as sharing blankets, so think about this when deciding the size. I think the double ones are really for people sleeping in a double bed alone.
When I got mine, Costco only had one size and weight in stock, although that may have changed as it was a while ago. I can't remember what the weight is, but I think it came in at slightly under 10% of my weight, and it is fine - I definitely wouldn't go heavier than you should. Mine is on the double bed where I go when my husband snores (ie most nights!) and it nearly covers the top of the bed, but when I am under it, so I'd guess at about 48" width, and 72" or so in length.
I can recommend the Costco one - it is much cheaper than a lot of them, comes with a removable cover that is washable and has one velvety side and one cotton side. The blanket has ties and the cover loops, so you can fix it in place to stop it bunching inside the cover. The brand is Kirkland, their own brand, which is always good quality.