If you insert a photo into Messages and hit Markup, the first view gives you a bunch of pen/pencil/brush options, with fixed sizes per brush. There's a plus button that allows you to insert shapes and text…but you can't pinch-zoom to scale the text. You then have to tap around again to set the font size and color.

It's a weird, neglected-feeling interface. They could borrow a lot from Instagram's story editor.