Yep, it's a simple volume question. You're not going to lose or gain any material, obviously, so the increase in length will be proportional to the ratio of cross-sectional areas.
3x3 = 9
2x2 = 4
Therefore, the length will increase 9/4, or 2.25 times, assuming you roll a perfectly accurate 2x2 square.
And I agree with David. Give yourself some leeway. Just like cheap tools are the most expensive, so is cutting too little material.