# What is the largest possible size for a brown dwarf?

What is the largest possible size for a brown dwarf before it has enough mass, capable of sustained fusion, thus becoming a star?

The mass threshold for this is about $0.075 M_{\odot}$. The boundary is a bit fuzzy - there is still some hydrogen fusion, but not enough to produce the luminosity of the object), it also depends slightly on the chemical composition of the object.
The plot below (from Nelson, Rappaport & Joss 1986 - newer models are available, but show the same behaviour) shows the radius of substellar objects as a function of time. The plateau at early times is caused by Deuterium fusion in object more massive than about $0.012M_{\odot}$.