I can ride the bike but its way too short for me (by big I asume you are refering to my hight
) . My own bike has the handle bars and seat at the very ends of their high adjustments (and a big A$$ pillowy seat, I hate the skinny ones up my butt crack
). I don't know where I would look for a frame size but to me it looks like what we as kids called a standard 26 bike. This bike also has a comfy seat , not a skinny racer bike seat. You may be able to get a size reference by looking at the relative size of the bike and grill behind it.
Oh , I also just thought of a detail. My sister was 5 foot 8 inches tall so that plus the seat height in the picture might tell you something about the frame size.