Take a look at patterns that are approximately that size and figure it out from there. If an afghan is 40x50 that would be 2000 sq inches, and if you want it 40x70 that's 2800 square inches which is 1.4 times larger. So if the 40x50 takes 2400 yards, the larger one would need about 3360 yards. You might need more if you use cables or ribbing as they draw in, but you might need less if you use a larger needle, like a size 10 or 10½.
Another way to figure it would be to cast on about 40 sts and knit 40 rows. Measure the sq inches in that, then measure how many yards it took (simple estimate is measure how much yarn for one row times X rows plus add an extra row to account for the cast on.) and figure it out from that.