I'm currently assisting my local Toastmasters Club by creating a series of graphics for social media, namely Twitter, Facebook and Instagram. They usually consist of an interesting photo and some text. Each one is sized 1200 x 680 px, which seems to work well. I've had feedback that some of the text is too small, and that people can't read it on a phone (without zooming in). How small should I go? The headline text is typically around 60pt, the body copy around 19pt. I don't want to make it too large as the contrast and hierarchy will be lost. Thanks.