Whilst it may come as a shock to the school that maintains that all women have the same sized vagina, it may be that you simply have a small vagina or vaginal openning. This is no less common than a man having a small penis. OK it may be slightly less common, but it is not unusual.
Certainly for now, I would advise that you are going to be size incompatible with an average and larger sized man. We spend lots of time here discussing the ramifications of not being sexually compatible with the average or smaller sized man and it is really no different for the inverse.
The plus side is that few guys are going to feel a bad rejection if you tell them that they are too big for you. Everyone has to find self acceptance for their physical self, so you are certainly not alone.
I know what you're saying, but for her to be unable to use tampons at all or insert a single finger without pushing hard it seems unlikely that it is simply a case of having a very narrow vagina. Were that the case, she'd need a man with an extraordinarily thin penis, not just small, thin. We're talking 2" circumference or less. How many men out there have that thin a penis?
Besides, it seems more likely that she has vaginismus.