A male is easy to spot as mentioned above, The female resembles ablue acara in the face and the colours on the male are unmistakable, The male also develops a nuchal hump or Kok, a big forehead bump basically . I recommend a male unless you intend to breed them as they are more sociable and active, females get very narky as they age (Don't they all!!! ) although every GT is different They grow to 14 inches or so although a female will stop growing permanently when she becomes gravid(pregnant) for the first time. This can happen at 5 inches. Growth and rate is dependant on water quality, diet and tank size. In optimum conditions they grow fast around an inch a month untill it's around 6 - 7 inches then it slows down to roughly an inch every 2 months , bigger it gets the slower it grows so it can take 3 years to reach 14 inches or more. I have seen males fully grown at 9 inches and also at 16 inches! (rare!). Genetics also play a role in ultimate size. Check out my male GT and tank in the link in my sig for an idea of what can be mixed with them.welcome to the wonderful world of GT keeping, One of the most responsive of fish you can own ,like having a dog in a fishes body! Big personality!