That can be done with the xterm options.

See "xterm -h" for all the options. To execute cssh in that xterm, use the -e option, like " xterm -fn 10x20 -geometry '16x19+0+20' -e CSSH @CSSHOPTIONS "

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = tkinit;

$mw->Button(-text=>"Xterm",-command=> [\&go])->pack;
$mw->Button(-text=>"Xterm with attributes ",
           -command=>[\&go, q(-fn 10x20 -geometry '16x19+0+20')])->pac
+k;

MainLoop;

sub go {
   
  print "@_\n";
  my $xoptions = shift || '';
   
   local $SIG{CHLD} = 'IGNORE';   # don't wait for child
                                  # exit status. avoids
                                  # zombies.
   die "fork failed: $!" unless defined(my $pid = fork);
   return if $pid;
   exec("xterm $xoptions");
   
   warn "exec failed: $!";
   CORE::exit(1);
}
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