When I try to post an image into a post like this one, it is actually 1024px x 877px but is not showing up full size even if full size option was selected.
Seems regardless to the settings it fires up at 300px...
This is a bug. In general, this must work as is:
1) Fullsize - insert image with original image's size (1024pxx877px)
2) Preview - insert image with predefined small size, which is clickable to image with original size.
Thanks for report Michael, we will fix this in Beta4!