If you haven't put the pattern into the Rapid Resizer yet, go ahead and do it. There's a step where you can crop all but the part that you want to resize.

If the pattern's already in the Rapid Resizer, you have to put it back again to re-crop it. Here's a quick method:

  1. Open the pattern in the Rapid Resizer.
  2. From the Edit menu, choose Copy.
  3. From the Edit menu, choose Paste.
  4. Go through the steps and you'll get to recrop your pattern.