Ok, there's a trick to insert some style (like a font size change) into text, but it won't work with a rollable table result. Consider using another template, like &{template:npcaction} {{rname=Title}} {{description=Text here}}, or just using the /desc Text here description block (/desc is only available to GM).
The other option is to get the players to install Stylus (style thread here) if they have trouble reading the fonts, and share the above code with them - each player can customise the appearance with this method (or not bother if they can read it OK). They can also do other things, like change colours around, if it helps with visibility of elements.
And a low tech, easy solution - players can also zoom in on their browser (and zoom out on the Roll20 map control) to increase all font sizes in the chat bar. It'll blur the token art if you zoom too much though.
FWIW I can read the {desc} and {traits} templates fine on a 1440p screen (I'm a big fan of the traits template... very clean, but the text is the same size as desc so I didn't recommend it above). So your players may not all need the text size increase.