My great dane is a little over 13 weeks now weighing in at 29 pounds... he was having a little trouble eating and compared to other GD's he seems small to me... I was just wondering what his size should be at this age...

What has the vet said? Puppies grow in different ways and at different times even in the same litter. The "trouble eating" is of more concern. Did you change his food or make any other big changes at the time?