No need to actually create a custom class - Blocs does it automatically. See below. Automatically resizing text is often a bad option. I have a couple of apps that do this, but it becomes a real pain adjusting text and layouts afterwards. This is because if automatic resizing is used. a 12pt text size on a desktop breakpoint suddenly gets reduced to about 5pt on a small mobile screen - making it difficult to read. It’s much better to have a system that maintains the text size and wraps the text to suit the screen width. You are then free to tweak the sizes if it looks too large on other breakpoints.
By way of example, the illustration below shows what happens in an app that automatically resizes text. In this case, it was created in Sparkle. You will note, if the text size is changed in the mobile breakpoint, the layout of the page will also have to be changed to reflect the larger text box size. This is where it becomes a bit of a pain, particularly if you have a lot of page elements. Basically, everything below the adjusted text would have to be manually moved down the page to accommodate the changes. This doesn’t happen in Blocs. The text box can expand when changes are made and everything below it will be pushed down the page automatically.