I have played this pack multiple times, with and without datapacks. Every time I've selected a Starter, it's always been sized as Small. I recently went into a creative world to test this. I selected my starter, and it was small. I then reopened the starter screen and selected the same starter, and it was small again. I repeated this process four more times, and they were all small.
NOTE: wild spawned starters vary in sizes, as they should. I adore the Pygmy Primarina I have in my survival world. There's no real issue there. the only issue is with those you acquire from the Starter Select menu.