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 scrollback_lines 500
Set history to 500 lines.
/set scrollback_time 1day
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