mencoder detect filesize

[nordic bro]

I know there's a mencoder option to preselect the output's filesize but does anyone know if there's a way when running mencoder for it to guesstimate the final filesize of what's being encoded, before it's actually finished?

I notice when encoding if you don't use -quiet or -really-quiet the progress seems to be showing what it thinks will be the final size (I guess that's what it's for) - is that correct? if so is there any way I could ping mencoder from within a script or something to return what that value is?