Puting your CCS in the child theme will work fine, but you will need to remember to port it if you ever change themes.
I prefer to put it in “Dashboard/Appearance/Additional CSS”, but if you never change themes it’s a moot point.
I’d add something like:
#w_quotescollection_3 { font-family: Arial; font-size: 14.5px }
Then press the “publish” button.
Before deciding which unit to specify (“px” in my example), I recommend reading:
https://kyleschaeffer.com/development/css-font-size-em-vs-px-vs-pt-vs
which explains the alternatives including their relative strengths and weaknesses.
An excerpt: “One problem with the pixel unit is that it does not scale upward for visually-impaired readers or downward to fit mobile devices.”
There are other useful CCS tricks to change the appearance. For instance, a border and a background image for the quotes. See KillerNoms.com for an example.
Good luck!