"Penny turtles" Do not exist. The myth arose from people who kept hatchling turtles in small cramped conditions and fed them the totally wrong diet and not enough of it. Chances are that the turtles died due to insufficient nutrition and very poor enclosures so never grew to adult or even sub-adult size, so people said they always stay small.
Also another myth I frequently seem to hear is that turtles won't outgrow their tank. i.e. keep a hatchling in a tank thats 30cm wide and 20cm deep it will never grow. Truth is it WILL grow and it will need a large tank or outdoor enclosure as an adult. My kreffts boy was the size of a 20c coin when I got him, and now being 2.5yrs old is about 25cm long (shell length) Most of the species in Australia grow large, and the males are usually the smaller out of the two genders.
Finally... Just as a note, turtle "breeds" do not exist; they are actually different species, not breeds. With dogs you get different breeds like German Shepherds, Border collies etc but they are all still the same species; Canis lupus familiaris. With turtles Kreffts, Murrays etc. are actually different species.
I wish you luck, however please thoroughly think it through before you get a turtle; they have long lifespans and are living creatures; they're not simply disposable. They are very expensive to keep and have higher maintenance levels than many other animals, such as dogs, fish, other reptiles etc.