My blanket is a baby blanket size, but the same pattern could be sized up easily by (a) changing the number of stitches or (b) using bigger yarn.
One of my reasons for making this blanket was to experiment with designing a quilt top. Thus, I hope you see this instructable as an invitation to design your own pieced blanket and learn about piecing without having to worry about 1/4" seam allowances (as you would in sewing).
Step 3: Make Triangles
Cast on 30 rib stitch.
knit, purl, knit, purl for 28 stitches. k2tog.
k2tog. knit, purl, knit, purl the rest of the row.
Repeat until you've reached the end of your right triangle
You'll need some squares too. Those are also 30st rib for 30 rows.