Around the same size would be your best bet. And putting them into a QT together is a smart idea, this would allow you to separate them easier if they don't get along.
I have had success with pairing maroon clowns. Got two the same size about an inch or less in size, they were small.
At first all was fine as they stayed on separate sides of the tank. It seemed almost as if they notice the rose bubble tip anemone at the same time.
This is the point where they started to scuffle. I did get nervous fearing I wouldn't be able to get one out in time to save it in such a large tank with full stock of corals. To my surprise just some tattered fins from the fighting and three days later they worked out a pecking order where the more dominant one hosted the huge anemone while the other stayed below it. Within about two weeks they both were hosting at the same time and enjoyed one another's company.
So it can be done, it will get to your nerves some as well.
Just remember that in a smaller tank like a Qt you can always be ready to put a divider in the tank or just remove one if you feel you can't watch the scuffles or one clown is clearly not going to make it through the aggression.
Hope this helps,
Good luck and happy reefing