Brug streng bredde for celle med
$string = fullname($USER);
$cellWidth = $pdf->GetStringWidth($string);
$pdf->Cell($cellWidth + 5, 0, $string, 0, 0, 'C', true);
String width doc http://www.fpdf.org/en/doc/getstringwidth. htm
Celledokumenter http://www.fpdf.org/en/doc/cell.htm