Frame size generally refers to the glass size or rebate size, unless otherwise stated. This will be bigger than what you see from the front as it includes the bit that goes under the rebate (the bit of the frame which overlaps the glass etc to hold it all in place. Eg. 30 x 40cm has glass size 300 x 400mm.
Plus sizes are designed to take a particular paper size with a mount. Eg. A4 Plus has a mount which comfortably fits an A4 piece of paper.
Sight Edge sizes are defined by the visible area from the front of the frame.