You may want to ask on the Perl-Gtk2 maillist for some real expert advice. From my amateur didlings, I would guess that since the table cell size seems to be set from the contents of the cell, looking at controlling content size and the various paddings mentioned in Gtk2::Table, would be the approach to take.
So maybe setting a certain font, and using Pango's font metrics methods would give you a way to control the cell size. See Gtk2 2d Table Demo and maybe look at the Gtk2 - Pango connection, Gtk2::Pango::FontDescription.
The newer Gtk3 may also have more Table controls.... it would be best to ask on the Perl/Gtk2 maillist.