Trunk Notes gives you a lot of power to customise how your wiki pages are displayed. However there isn't currently a nice user interface to this - you need to modify something called a CSS stylesheet.
1. First you need to enable the custom stylesheet: Tap the action button, select Settings and then turn on "Custom stylesheet"
2. Now go to the page Special:SetStyleSheet (tap the page name to show the search is the quickest way and start typing Special:SetStyleSheet). At the top of the stylesheet you will see the line:
font-size: 16px;
3. Try changing this for a different value. If you set it to 20px you will now find the body text in your wiki pages is much larger. For example the start of Special:SetStyleSheet would look like:
body { font-family: Arial; font-size: 20px; background-color: white; margin: 0; }
You can separately change the size of headings (h1, h2, h3) and other sections of your page.
By exploring Special:SetStyleSheet further you can change colours, add borders and visually spruce up your wiki.