I've just added subtasks for the first time using the new UI and found the font size in the Notes field almost unreadable.
I'm using a 24" widescreen monitor at 1920x1080 resolution (standard for that size monitor) and I haven't zoomed my browser window.
I think it's made worse by the fact that the font in that field during edit is courier. Can you perhaps bump the font size up a notch or maybe change the edit font to arial?