Hello,
Thank you for your message.
Unfortunately, Remote Utilities only uses the system font size, it doesn't have any own font size controls.
It should be noted that there are two similar (yet not same) Windows settings - make text bigger and make everything bigger. Here is a screenshot:

You should use the first option (slider) to increase text only. Since RU follows system settings this will increase the text size on controls in Viewer/Host dialogs.
Hope that helps.