I'm not 100% sure what you're asking. It sounds like you want the size of the UsedRange? If so, have a look at the Count property of the Rows and Columns collections of the UsedRange object (How's that for an awkward sentence).

It should look something like this:

use strict;
use Win32::OLE;

my $ole = Win32::OLE->new('Excel.Application', 'Quit') or die "$!";
$ole->{Visible} = 1;
my $xls = $ole->{Workbooks}->Open('mr.xls');
my $sheet = $xls->{Sheets}->{Template};
print $sheet->{UsedRange}->{Rows}->{Count}, " rows, ", 
      $sheet->{UsedRange}->{Columns}->{Count}, " columns used.";

__END__
Output:
C:\>perl xls.pl
40 rows, 11 columns used.
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