Google failed me on this. I'd like to lengthen my lastlog size in irssi. I know that it's possible, since I've managed to do it years ago. I guess its one of those more esoteric commands?
Set history to 500 lines.
Keeps at least one day even if it exceeds the maximum lines.
And here's the settings page for irssi: http://irssi.org/documentation/settings