The recommendation I've seen is 10% of body weight + 1 pound for a twin sized blanket (the top of the twin mattress, not hanging over the side of the bed) - scale it up for a larger blanket with the same ratio of weight to size.
My blanket is ~25 lbs, and about 60" x 80", I weigh about 150 lbs. (This means that the twin sized equivalent with the same weight per area would be ~10% of my body weight). This weight works well, especially because if I really want more weight I can put two layers of it on me.
That ratio works well for me, though if I get a second blanket I'd have it be smaller and heavier per unit area because I'd want it to be different and wouldn't want it to be lighter - I'd put it so that it'd be between 10% and 20% of my body weight per twin sized mattress area, so that I'd have the three different amounts available, my larger blanket, the smaller one (which I'd get for being able to bring with me places easier), and two layers of the larger blanket.