Changing font size for .editor-preview

[rlyfun]

How do I edit the font size of the text once I toggle the preview button?

.editor-preview,
.editor-preview-side {
padding: 10px;
background: #ffffff;
font-family: Arial, Helvetica, sans-serif;
font-size: 15px;
overflow: auto;
display: none;
box-sizing: border-box
}

I've tried editing the simplemde.css file
and I've also included a snippet in the html itself referencing editor-preview, however have not seen any progress.

Changing the font-family to Arial, Helvetica works in the code above.

However, changing the font size of the rendered text doesn't seem to be working as intended. What do I need to edit for this to work properly?