To make them like the ones we had from the street vendors in La Paz, Baja, they'd be about 1 inch square and about 1/2 inch thick. They put 3 pieces about that size in each tortilla, then you went around to other the side of the cart which was three stepped shelves with at least a dozen different garnishes and condiments that you could mix and match as you liked.
I've had fish tacos in a lot of US restaurants, and I've had some that were very good, but never anything that could quite compare to what we got in Baja. Those taco wagons also offered shrimp, octopus, squid... all battered and deep fried, and at that time back in the mid 90's, 7 pesos ($1) each.