I have a similar form that they are always completing via iPad, so I had to force it to always be drawn, and make the signature box bigger on screen.

Be sure to set it on the Layout Page to always "Prompt to draw or type signature".

Here's the javascript I used to force the drawn signature:  

$(document).ready(function () {

  //signature fields must be drawn and not typed.
  $('#sigNav a:eq(0)').hide();
  $('#form-signature-dlg').on('shown.bs.modal', function(){
    $('#sigNav a:eq(1)').click(); 
    $('#form-signature-dlg').draggable({handle: '.modal-header'});
    //$('.sigTabContent .form-control-box').dialog({draggable: false});
  });

});

Here's the CSS I used to increase the size of the signature box: 

/*resize the signature draw dialog box for use on iPad*/
.modal-dialog:not(.override-forms) {width: 800px; margin: 30px auto;}
.form-control-box {max-width: 800px!important; height: 280px!important;}

These are tested in Forms version 11.0.2212.30907 Classic Designer.