Weaned rat is not the smallest size rat. Daybreaker got the scale right:
They are called weaned rats because they have been weaned off their mother's milk and are eating regular food.
You can go here: http://www.frozenfeeder.com/sizing.html for a good conversion chart.
Some snakes switch right away and some are picky. There are a few methods for getting them to switch. You can try the "hungry" method where you wait it out and only offer rats till they get so hungry they take it. This takes a lot of willpower. There's the "scenting" method where you scent the rat with either a mouse or soiled mouse bedding in order to sort've trick your snake into eating the rat. I've used this method with quite a bit of success. I either thaw the rat in the same bag with the mice or I take the mouse and rub it all over the rat.
Just because a snake takes live once doesn't mean they're necessarily stuck on live for life. I've got a few that prefer live and will take them more readily, and then I've got some that will eat anything warm put in front of them. I've got an albino that I'm trying to switch to ft. He takes live great, but it takes quite a bit of the "zombie rat dance" to get him to take the ft. I have another who took live for me last week and ft tonight. So it really depends on the snake.