Here is a pic of mine (i bought 3 about 6 months ago) lets say they where an inch or a little less and now they are not that much bigger. I would say only one has doubled in size and the other two are maybe at an inch and a quarter.
Hope that helps with your growth rate inquiry. At least mine seem to be slow growers, guess ill just sit back have some
and wait. Despite their size they have a great color and I personaly love adding red to the tank.