Is this a signature field (without a signature applied) or is this your actual signature? In the first case, select Tools >Forms>Edit, then select the field and resize it. If the signature is already applied, you cannot modify the document anymore without invalidating the signature. Open up the signature panel on the left side and right-click (or Ctrl-click) on it and soft to clear the signature (you can do this for your own signatures). Then resize the field and sign again.