I would think it depends. If you mean to remake it all in stunning 3d HD then a lot larger. If you leave it really blocky (but 3d) then perhaps not that much...
depends on the textures mostly, also hed need to write his own (simple) 3d engine which will require lots of boilerplate. If hed use something like Three.js, that alone would be more than the size of the game currently i guess.