Signature Picture Frame mat size for 150-175 guest

sas0402

I am looking around at ideas for a photo frame for guests to sign... I see that the recommended size for a mat for 150 people is 16x20"mat (8x10 pic)... but then i read a review and someone posted it was a perfect size for 45 guests!!  Sooo... what size mat should i be looking for...  I would like to use the 8x10 picture. 

thanks