This would be a good feature to have - i.e. to preview a line of text across a range of sizes.
I’m hesitated to add any preview capability that goes further than what the preview view. Because otherwise I would need to re-implement a small layout app to cover all possible requests. And previewing proper text is especially difficult for complex scripts (like arabic and indic scripts).
So, as mekkablue suggested in another thread already, I recommend setting up test documents in Indesign and if you export regularly to the Indesign Fonts folder, you have almost live update.
Testing in a page layout program is the ideal way to do it because of all the control you have over the text. The Glyphs app couldn’t possibly implement all those page layout features without becoming a bloated, inefficient and difficult to maintain app. InDesign is really the best way for testing.
I will try the InDesign test. I didn’t think displaying a line of text in sizes that get smaller was technically complicated.