There are a few ways of going about this depending on what you want to change. You could start with these (adjust the values as desired):
/* body font for Editing and Reading views */
.is-phone :is(.markdown-source-view, .markdown-preview-view) {
font-size: 1.2em;
}
/* sidebar font and smaller font in search & backlinks results, etc */
.is-phone {
--nav-item-size: 18px;
--font-ui-smaller: 15px;
}