Just from checking the docs it looks like:

  $dc->{xsize}; # X size in chosen units
  $dc->{ysize}; # Y size in chosen units
[download]

Are the parameters you want. (And the default unit is inches).

Also, I don't know if your size is a standard named size, but it seems like many of those are supported via the papersize attribute.

Maybe this will get you started.

#! perl -slw
use strict;
use Win32::Printer;

use constant {
    WIDTH   => 7,
    HEIGHT  => 8.5,
};

my $prn = new Win32::Printer(
    ## Page size in inches 7" wide, 8 1/2" high
    unit => 'in', width => WIDTH, height => HEIGHT,
) or die "$!, $^E";

## Used to position the text vertically
my $hieght = 0;

## For various different font sizes
for my $fontsize ( 10 .. 30 ) {
    
    ## Select that font
    my $font = $prn->Font('Arial Bold', $fontsize );
    
    ## CALCRECT return the hieght of the text to be drawn (without dra
+wing it).
    ## CENTER the text horizontally between 0 and WIDTH (in units from
+ above)
    $hieght += $prn->Write( 
        'Hello, world.', 0, $hieght, WIDTH, HEIGHT, CENTER | CALCRECT 
    );

    ## So use that to position the bottom left corner
    ## Then draw the text for real
    $prn->Write( 
        'Hello, world. ' . $fontsize, 0, $hieght, WIDTH, HEIGHT, CENTE
+R 
    );
}

## Start a second page
$prn->Page;

## Draw a few rectangles to fit the paper
$prn->Rect(  $_, $_, WIDTH - ($_*2), HEIGHT - $_*2 ) 
    for map{ $_ /2 } 0 .. WIDTH / 2; ## Steps of 1/2 inch

## Close and print it.
print $prn->Close;
[download]