We have two yearling females that are crosses. The one is small and black, nothing to look at but bigger than a Dexter her age... The second yearling female is red and as big as an adult Dexter cow, not a huge difference in her from the straight females we kept except maybe a bit better put together.. She was an accident but she has grown..

We had a half Dexter Maine bull that we kept around for a while.. I think he was 1000 pounds as a two year old when we sold him.. Nothing but grass and hay... He just got a bit ornery.. He was fatter than a pig and half blind in one eye. Should have butchered him but took the cash instead.. We sold a Straight Dexter bull that was same age as him and he was only 700 or so.. 

I would imagine a 1/2 blood bred back to a "normal" breed would throw a good critter but the bigger you get them the further they are going to get from Dexter meat, whatever that is (And yes, I do think it has a different grain to it than the larger beef breeds).