The estimates I read put an upper limit of around 4000 KM on it, but if they're now thinking it could be upwards of 9,500 KM, that puts it at Mars-sized. Pluto's definitely about have an identity crisis.
The problem is that they have no clue what the albedo of the object is. It probably will be more on the order 4000 km, but it will take a while to make that measurement.
The other monkey wrench is that this particular object is not really a trans-neptunian object like Pluto and others, but something that is sitting somewhere between the Kuiper belt and Oort cloud. Lacking experience with objects in that part of space, they do not have any baseline averages of what the albedo likely is.