I would say that this diamond is a 1.5 carat. Remember that cut is the most important "c" of all. A well cut diamond is everything. Can you give us a rough budget and see if we can find a diamond that might be good for you?
Its not going to matter what size that diamond is if you dont know what size your gfs finger is. I dont think you mentioned that? If this is a 1.5 on a size 4 and you get a 1.5 for your girlfriends size 7 shes going to tell you that your diamond is no where near that picture of a diamond she showed you.
As far as the assumption on clarity/color goes you can make an I look like an E with an ideally cut nearly flawless diamond. There are really numerous combinations to get what you''re looking for. The only way we here on price scope know pictures of diamonds are said cut/clarity/color/carat is because people post those stats. They could bump up the carat size, the color, or the clarity and we wouldnt be any the wiser unless it was a blatant lie.
SailorSweet is right--your gf wants a diamond that has that same finger coverage as this one, so her ring size will totally dictate the size you need to be looking for.
Well, perhaps your budget should be the first indicator, but then consider her ring size.