Platies come in several species but the common varieties most often found in the pet shops are usually the ones that can grow bigger, up to about 2.5 inches. (You will see a range of 2 inches to 2.7 or even 3 inches listed on different sites.)
Since your tank is about 6 US gallons you have about a 6" allowance of fully grown adult fish body and you will have less surface area for oxygen in your unusual tank. Your two fish will put you at about 5" and if you take away a half inch for the oxygen problem you are probably fully stocked. (This is why many of us older hobbyists think of tanks like that as "starter" tanks, as they kind of get you interested in the hobby and quickly make you realize you want a bigger tank!)
~~waterdrop~~
