Depends on the ring of course, but I had one done several years ago. Stock piece at a jewelry store and I loved it. Too big so it was sized down for me.
Like all kinds of rings it just depends on what kind of setting you are looking at. If it is a plain shank you can probably do that large of a jump. Do you have a picture?
It really is hard to say without seeing it though. I know that sounds bad. In general my jeweler doesn't have much issue going around 2 sizes with almost any ring I've brought in. Some rings are so ornate though, that they can't do more than 1 full size really. Plain shanks I've had them go down from a 10 to under a 5 without any issue at all. I will admit that I never buy rings that are not w/in one size of my ring size unless I am buying straight from a jeweler who can guarantee they can resize it for me.