One of the mechanisms for the heating of asteroids in the early history of the Solar System is believed to be decay of the isotope ${}^{26}\mathrm{Al}$. This was created by the supernova that produced the dust cloud from which the asteroids formed. For example in this paper the astrophysicist G. Jeffrey Taylor wrote:
${}^{26}\mathrm{Al}$ was present when meteorites were forming (see PSRD article Using Aluminum-26 as a Clock for Early Solar System Events). It is a radioactive isotope with a half-life of only 700 thousand years, so its presence means that the solar system formed within a few half-lives of the formation of ${}^{26}\mathrm{Al}$ in an exploding star. It decayed by emitting a beta particle (an electron), creating ${}^{26}\mathrm{Mg}$ (magnesium-26) and releasing energy. The energy released is considerable. If ${}^{26}\mathrm{Al}$ made up only $5 \times 10^{-5}$ ($0.005\%$) of all the aluminum in a chondrite (most is aluminum-27, which is not radioactive), it would release enough energy to melt asteroids a few kilometers across and larger. Lower amounts of ${}^{26}\mathrm{Al}$ cause less melting.
(my emphasis)
I have been trying to estimate the minimum radius that the asteroid would need to have for it to melted due to radioactive decay but I do not know how this calculation is done. Can anyone describe how this calculation is done and how Taylor arrived at the result a few kilometers across and larger?