Figure you need enough room for him to turn around as an adult... if he's 3' long, he won't be able to comfortably turn around in a 3' wide tank. Plus, he can't swim very far lengthwise, either. That's like keeping a neon tetra in something 3"longx2"tallx1.5"wide.... not a whole lot of room.
Can you make it like 4'widex10-12'longx3'tall or larger?