Teacups (reticulated) will get rather large and you will need a large tank to house them in as you already know. I think that the tank with the dimensions you listed would be just fine. It's recommended to keep them in nothing smaller than a 5ft tank when they are about 5-6" and you could keep two of them in there together. Long term you will need a tank with the dimensions you listed for a pair. I think females get a little larger than males. They aren't for beginners as already mentioned. They can be costly since most of them will require exclusively live foods like ghost shrimp, blackworms, earthworms, feeder fish, etc. The bad thing about these fish is that they are inexpensive (about $40-50 for a pair) so ANYONE can get them and they aren't for beginners. If you think you are up to the challenge and can handle caring for them then go for it. I'm not trying to say that you aren't experienced, just pointing out that they require more work and care than many other types of fish.
