This would likely be dictated by your theme styles.The theme your using will hopefully have some customization enabled. if it does you can go:

Edit Ribbon > Gear Icon > Design > Customize Theme (Will be a button below the selected theme there if enabled and available) The panel looks like this:https://documentation.concrete5.org/developers/designing-for-concret...

If the theme designer has set it up, you might have an option in there already to just change the font size of form elements but maybe not.

At the bottom of that customization panel though would be a CSS text area box where you could apply some rules like



You might also be able to click on the form block itself in edit mode and chose Design & Custom Template. The first choice there "A" is some typography settings but they may not reflect if your theme is using like a bootstrap UI kit or similar because it's explicitly targeting those input form elements rather than the outer wrapper of the block itself.

Outside of that you could get into creating a custom block template where your template has its own stylesheet. Find more info here on that:https://documentation.concrete5.org/developers/working-with-blocks/w...