Step 1: Navigate to the Configure tab and click on Text Boxes. You can see the current length of each field in the Length column.
Step 2: To edit this length, click on the pencil icon of the field you want to edit. The line will turn green and the information will appear in the fields above. Click the Length drop down and choose the desired length. Click on the green check mark to save the field.
Step 3: Click on the Input tab to see the change immediately.