Can you change the font & letter size of the place setter words shown in the fields?

You could do it by adding some CSS code, but you need to first import the font style you would like to use, upon checking your form I can see you're using "Josefin Sans" and "Sans Serif", to import a font you do it like this: 

@font-face {

  font-family: 'Josefin Sans';

  font-style: normal;

  font-weight: 400;

  src: local('Josefin Sans'), local('JosefinSans'), url(https://fonts.gstatic.com/s/josefinsans/v9/xgzbb53t8j-Mo-vYa23n5ugdm0LZdjqr5-oayXSOefg.woff2) format('woff2');

  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2212, U+2215;

}

Then you only apply the font to the text boxes and text areas of your form, this code is an example about how you could do it: 

.form-textbox, .form-textarea{

font-family: 'Josefin Sans', sans-serif;

}

This guide will help you to inject the code: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

Here's also a cloned form of yours where you can see the result: https://form.jotformpro.com/70855127470962 

Hope this helps.