A starter reef tank... Their are quite a few tanks that are perfectly capabale of supporting a reef tank. As others have mentioned bigger is easier or at least more forgiving of lack luster maintenance. With that said, a smaller tank lets say something like a nanocube or biocube in the 24 gallon range is still considered small but will have lots of the things you need to start a low light reef. The key is maintenance PWC
's and Top off's if you are diligent in your schedule you should be able to make any size reef work.
Research it first. While a 55 gallon tank would be a good starter tank if you have the room but the initial costs could be daunting. Where a smaller All in One tank might be a less expensive option but will limit you.
I would say ask your self this "how much time am I willing to commit to this endeavor?"
Like the others have said pick your tank and then we can certainly help you from there.