I own a 10 month old marginated tortoise who is 6cm long and 3cm wide is that normal?
Reptiles in general don't grow the same way mammals do. Every single one of them will grow at different rates depending on how much they eat, what they eat and their environment. If he grows slowly it doesn't necessarily mean he's unhealthy, you will often see sibling tortoises be completely different in size (one can be double the size of the other) .
Keeping a chart to monitor growth is ideal, there should be a steady weight gain in correlation to his length. So I would say yes it is normal, however pictures would help us see what his overall appearance looks like.