I've never really tried to correlate any particular toy size with fist size. Over the years, I've spent a hell of a lot more time playing with toys than I have getting fisted. Damn the bad luck! But, since playing with toys is fun too, I've just always played to get the maximum stretch that I could get with relative comfort. Any time you're training a muscle, there will be some discomfort. Just don't cause yourself any real pain. Pain is your body's way of saying "don't do that."
The important thing is to keep training, keep stretching, keep playing. You're training your sphincter to relax, giving it flexibility, making it ready to accept your lover's fist.
And when that time comes, the most important thing is trust. You have to trust your partner 100% to be as dedicated to not hurting you as they are to successfully fisting you. Expect a little discomfort, a fist is shaped differently than a toy, with few exceptions. But know that your partner has your best interest foremost and will stop if you ask them too.
Now, just lie back, let the sensations from your ass take over your mind and prepare your ass to open and welcome your partner's fist into your body. It probably won't happen immediately. It may take several sessions. If that's the case, so be it. Just don't get in a hurry and do yourself permanent damage. Success is worth waiting and working for.
When you finally feel your ass relax and let that last knuckle in, you'll be overwhelmed by the feelings and sensations of the most intimate thing that one person can do for another. Treasure it.
ldb