How can I make the Star Rating bigger with CSS?

I'm afraid that can't be adjusted since the Star Rating field uses image sprites. You can find the image used for all the star states by clicking this link.

Perhaps using a widget that is similar to a rating scale would work as an alternative.

1. Knob Widget - https://widgets.jotform.com/widget/knob

2. Gauge Input - https://widgets.jotform.com/widget/gauge_input

Related guide: How-to-Add-a-Widget-to-your-Form

Give those a try and let us know if you'd need more help.