What are the Pros & Cons of sticking to default Story Size vs. going larger?

I hate to contradict Ashley but the player will change based on the slide size as the buttons will get smaller and progressively more unusuable (not an issue if you do not use the player buttons), Also in HTML5 the font size of the player and menu cannot be adjusted so that may be an issue.

I would suggest that in an environment where you cannot control the devices using your courses 1600x960 is too large.  Tablets using this resolution will struggle to download the content and it may cause them to crash, the ninja game I built was originally 1080p and crashed iPads on the first slide (android devices faired a little better they got to slide 3). I would agree with Dave a lot of users now have much smaller screens (although have higher resolution) than perhaps 5 years ago.  Also the larger the project the higher the bandwidth requirements.

I generally go for 960 - 1000 pixel width for most projects and let them scale, the one caveat is that if I am using screen recordings or screenshots I would recommend locking the player as scaling will pixelate your images.

Articulate recommend that if you are targeting iPads you use the 720 width, I find this a little small.