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Needed to do this myself so was just checking the most recent version of FPDF and it looks like the width & height are already available as public properties. So for anyone looking for the same info:
$pdf = new FPDF();
$pdf->addPage("P", "A4");
$pdf -> w; // Width of Current Page
$pdf -> h; // Height of Current Page
$pdf -> Line(0, 0, $pdf -> w, $pdf -> h);
$pdf -> Line($pdf -> w, 0, 0, $pdf -> h);
$pdf->Output('mypdf.pdf', 'I');