The Mark as Read button is probably the most-used commonly button on the whole site. Hence, I think it makes sense to make it a larger button, preferably a little taller and definitely wider.
You know about shift+a?
Yes, but I’ve found that for reading stories for me is primarily a one-handed, mouse-driven activity, done from a relaxed posture. Hence, I was focusing on the clickable interface.
The buttons that are most commonly used should require the least fine-motor skills and attention to access. I’ll contrast it with the “Original/Feed/Text” buttons, which are 4 times or more larger, but accessed much less often. Similar with the Split/List toggle.