@discoverthefire in order to change the font-size via your child theme you will need to add a new entry into your child theme’s main CSS file. I forget if this is called custom.css or default.css in the child theme.
Add this:
html, body, div, span, applet, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, a, abbr, acronym, address, big, cite, code, del, dfn, em, font, img, ins, kbd, q, s, samp, small, strike, strong, sub, sup, tt, var, b, u, i, center, dl, dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td {font-style:arial; font-size:14px;}
You can edit the font settings as necessary