To make a large blanket fast you'll want to do big yarn and big needles. You can use US 35s with three or four strands of worsted weight yarn held together. The yarn depends on if you want it washable or not.
I'm making my first afghan now. Not sure how big you mean by 'adult size' but I'm doing 200sts in a feather & fan pattern (4 out of the 6 pattern rows are all knit, one purl row and one feather & fan row, so in other words fairly fast and uncomplicated knitting) with worsted weight yarn and size US8 needle. I started it a month ago and have been consistently working on it and I have approx 2ft of knitting done. I'm on my 4th skein of yarn out of the 11 skeins that I plan to use. It will be a total of about 2300yds of yarn.
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Ravelry username: HeatherDD
There are lots of options on making an adult size blanket.
We are making three Oddball Charity Blankets that will all be full-size. For those blankets, we are using worsted weight yarn, with a size 8, 9, or 10 needle. We cast on 300 stitches, and knit until it is long enough. It should be at least 60 inches by 80 inches for a full/queen size blanket.
I should have said, I started to make a larger style throw rug (which would be like something to throw across the bottom part of a double bed (not pillows). It was knitted in five panels with changing yarns and textures etc. It looks great BUT what has been the killer is weaving in the end. There are scores of them..scores..and I haven't yet finished the work simply because I keep getting blah about that. Because then I have to sew the five panels together. It will be wonderful when done (tho I already wish I had done the edges a little different but those ends! So, I would now do something where it was a continuous knit - perhaps panels - but little colour variation and so on.
But the squares Shandeh's patterns had are great. True a load of sewing together also but at least you see the squares pile up :-)
Otherwise I would go for big needles like sarah_jayne suggested and a rich yarn and just enjoy. Again it can be yarn choice. I'm seen some superb bed throws just simply knit with big needles but yes, more than one strand of yarn SO you need to consider cost.
I made my Husband and I a double bed sized blanket. I used DK yarn, and knitted squares 12" by12". I crocheted a boarder around it then. I did all sorts of different stitch patterns (all in the same colour) and put them together randomly.
We use it every winter, we used to have two duvets in the winter but this blanket keeps us nice and snug.