Hello,

You can use custom css to change the size of  jsignature signature field.

In Form Layout tab, you can select signature field to configure. There will be two tabs when you select signature field. In Advanced tab, you can first give custom class name to the signature field. Then you can apply custom css (probably, width and height) to div under the custom class (like .custom-signature-field div). To change the size of img, you can apply custom css like (.custom-signature img).