Let's face it, i'm hardly a typical woman when it comes to such matters, am I? I choose to sit a similar distance from an 80in projector screen!
But I may be able to help.... My Dad's going through the same arguments with my Mum re TV size: they're replacing an old 32in CRT with a 40in LCD - I advised Dad to get the measurements of both. Bingo! The 40in LCD is the same size.
Maybe by cutting a piece of card/paper to the same dimensions of the 50in set you have in mind, and sticking it where in the room you want it to go will show that it's really not that big at all - sounds a bit Blue Peter, but it could work.
Failing that, wax lyrical about how great her favourite shows are going to look - all the extra details, drama, richness....