Fireworks is not completely compatible with retina screens. I think that causes the problem for you, since Fireworks will interpret the resolution twice as large, it displays it at twice the size compared to the actual screenshot.
You can test this by taking a small screenshot (command shift 4) and comparing the relative size it is displayed at in Fireworks.
Also, zoom out to 50%: does it match your screenshot size?
If so, unfortunately the only solution is to zoom out to 50%. Fireworks is dead software, and no longer developed or updated.