I have grown an apricot from stone, now 6 feet. Will it fruit, and how big will it grow? Could I use it to graft other apricot onto?
Graft or Not?
There are many varieties of apricots, so it's hard to predict the behavior of your particular plant without knowing the type of apricot it is. Here is an article about apricots in general: https://www.gardeningknowhow.com/edible/fruits/apricots/apricot-tree-growing.htm
You can use any rootstock for grafting. If your plant seems to be very strong, e.g. cold resistant, drought resistant, or whatever, you might want to use it. to graft a more delicate apricot variety onto. Here's an article on grafting: https://www.gardeningknowhow.com/ornamental/trees/tgen/grafting-trees-what-is-tree-grafting.htm