In order to retrieve the form signature with the desired size in the generated PDF, you can use the SVG Generator in the Convertor option of PDF Generator. DOMpdf converter doesn't support size for signature.
For creating the SVG file, it is recommended to use Inkscape which is absolutely free software. Otherwise, you can also use our online SVG generator tool here
You can use a rectangle for signature and give it the ID using a form token.
Once done with your template, navigate to View > Source or simply use the shortcut Ctrl + U to copy the source code to be pasted in PDF Page settings of Form PDF Generator.