You can change the font size and style of the entire form by clicking on styles tab. 

You cannot update font style and size on one control unless you use CSS - to my knowledge anyways. The only thing you can change on a given control is border color, control fill and control input color, if not using CSS.

Here is the CSS guidelines should it help! 
Custom CSS Guidelines (nintex.com)

Here is an example: 

Large green font for only my first text control. 
 

Under the css code editor my code is:

sdir] .nx-theme-styles .custom-test label {
  color: #17a80b; 
  font-size: large;
}

On the control itself under CSS class I titled it “custom-test”.

Hi @AdrianGaribdass01

 
@brandiwoodson Is correct, there are some basic abilities to control font and text size for labels and input in the Styles tab:

 

If you want to add a custom font then you can make use of CSS by enabling the advanced styles:

 

And you can then use a link to load the font you want.

 

@import url('https://fonts.googleapis.com/css?family=Montserrat');

ntx-form-runtime * {
    font-family: 'Montserrat'!important;
}

Then you will see the font applied:

@Jake @brandiwoodson 

Hi Jake/Brandi,
Apologies for the delay, and thank you for your response. I looked at activating the Custom CSS feature but wanted to verify if the incompatibility with future form upgrades applies only to the individual form that has the feature enabled or to the entire application. Do future upgrades impact all forms?

I ask because I first have to enable Custom CSS in the settings menu, which makes the option available in the form, and then enable the CSS feature under Advanced Styles in each individual form to change the style. Making the entire application incompatible for future updates does not make sense to me. 

Hi @AdrianGaribdass01 

The message regarding the upgrades potentially stopping the CSS from working is only really there to cover from an accountability aspect from Nintex, Ultimately customisations like that are your choice and Nintex support may not assist you.

There are a couple things you can do to make administration easier. 

If managing multiple forms is an issue then you can always host the CSS file and use the link feature:
 

That way if the CSS needs to change for all forms you can just update the single file and all of the forms will pick up the change, hosting the file is entirely up to you, most companies will have some kind of public hosting capability.

The CSS I shared only contains one potential issue which is if we choose to rename the tag ntx-form-runtime which I would say is quite unlikely but not impossible. 
 

Since its implementation I have made many CSS styles for our forms and I am yet to encounter issues but ultimately the choice to use it is entirely up to you. 

Jake