7" is plenty for a try at breeding her. You need to feed her well first so that she does not want to eat the male out of hunger. With this species you can leave the male in the tank with her for a long long time. I have mated this species once, ( I never got a sac ) but the male I got was on his last leg anyway.
She might take food now if its been 3 weeks. Just offer something every now and then and if she does not take the item within a few hours remove it and try again another day.
Alot of good facts and tips where posted so here's my 2 cents:
Fact: Size technically doesn't matter when it comes to tarantulas being able to be bred. It more of a common sense scenerio. (Like trying to mate a 7" male with a 2" female . I personally think that would not work)
Scenerio 1: If the male is too small when he matures compared to the female he might not have the strength to lift her to make insertion or escape a non receptive female and will likely become a meal rather than a mate. The above is true for the female but, is highly unlikely
Scenerio 2: If the female is bred at a small size, she will probably molt before an eggsac can be made and will likely need to be bred again because she would have shed any stored sperm she obtained prior to the molt.
To answer your question about breeding: I would try and locate a freshly mature male and try to breed her. At 7" she shouldn't have any problems mating IME.