Seals images included in the certificate module are 80x80.
But if you find default seals too smalls or too bigs, you can use images with other dimentions and the code will not resize them (assuming it has not been modified !). There is, I think no maximum but if too big, it will hide/overlap others elements (as seal is at the lower right corner it can only hide borders and lines) and be cropped at page margins.
If you create a customized mod/certificate/xxxxxx/certificate.php, you can change the seal location by editing the lines
$sealx = 150;
$sealy = 220;
(These lines are from the latest version of the A4_embedded certificate numbers can be differents in other certificates and other versions)