My RE prefers a lining of 8 mm or more. Not sure what the absolute lower limit would be, though. A 5 does sound low, but like adgal said, that could have something to do with your miscarriage already impacting the lining.
If this is a recurrrent issue for you, it can usually be corrected with fertility meds in the follicular phase (before ovulation)...either with an estrogen supplement or an ovulation inducing medication, since both will increase your estrogen levels.
I would definitely call and check with your doctor, though, to make sure you know for sure what the number was.
Good luck!
I don't know the answer to this, but I googled it and here's what I found....up to ovulation the normal thickness is 5mm. Post ovulation it's 6-15 mm. I would think that during a miscarriage you are shedding lining as well, so I would think that your number would be fine. I am not a Dr. though, just trying to help puzzle it out for you. I would make a follow up appointment and discuss it with him/her. Best of luck to you.