My blocks always come out to small until I reset my machine to even less that a scant 1/4 in. If I do a full 1/4 they are way to small even though I measure to make sure it is 1/4 in.
Now I set my machine for 1/4 in, then adjust my seam to 3.0. Now my blocks come out perfect. Each machine is different so you have to experiment. The strip test that Pam mentioned is a good way to measure.
I have used 1/4 inch feet but it still dosnt come out right.