When women wear the GIA Certified Diamond on the ringfinger of their left hand it is because the Egyptians claim that the vein of love goes from the heart and to the tip of this finger.
Flawless Tiffany Style Diamond are no more beautiful to the unaided human eye than a grade in the middle of the scale. Once again, extremely subtle differences in clarity are very important to determine rarity, but only affect beauty when in the very low end of the scale.
While the weight of engagement ring is important, it doesnt necessarily provide the "perceived size" of diamond store. A shallow cut diamond will have a greater diameter, and will therefore look larger.