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.
There are also lots of free patterns online for bedspreads, blankets or afghans.
Pomander Walk Bedspread
Square Counterpane Bedspread
Chevron Stripes Bedspread
Squared Off Bedspread
LOTS of afghans