The editor font size is set in Joplin's settings. CSS that sets the main text elements of a note in the viewer are as below. Change the pixel size to meet your needs.
body {
font-size: 12px;
/* sets body text font size */
}
table th, table td {
font-size: 12px;
/* sets table cell and header font size */
}
code {
font-size: 12px;
/* sets code font size - fenced code and pre */
}
.inline-code {
font-size: 12px;
/* sets inline code font size - code element does not seem to control this */
}
The different elements have been listed separately so you can add any other changes you may want to each one.
These threads on the forum have other examples of using both userstyle.css and userchrome.css