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My colleague already answered your question in this thread: https://www.jotform.com/answers/830328-How-can-I-for-only-ONE-textbox-set-the-text-align-font-weight-and-text-height-Tx
Basically you will have to have ID of the field that you want to add Custom CSS to, and then you can change font size, font, color etc of that field only.
ID of my field is input_3, yours is probably different, in CSS below I set color of the text to be red, font size 30px and font family Open Sans. Feel free to adjust to your needs
font-family: "Open Sans", sans-serif;
font-size : 30px;
Also check this guide on how to Inject Custom CSS to your form.
Please let me know if you have further questions.