Anybody?

Kellie
Does anybody know is 3mm a good size sac? After being told I had a miscarriage last week they have found a sac! But they are worried I may still lose as my hcg levels are only 1225.. Just want a bit of hope I think I'm 8 weeks but they think I'm maybe only 4/5 depending on size of sac but is it a good size for 4/5 weeks if that's what I am?