Hello mate, they seem to max out around 20"-22" but can push 2 foot. I think a 6x2x2 should be ok for 3 years but then a wider upgrade will be needed. Like most big cats they spook if wiskers touch sides both sides of tank.
£250 is abit pricey for a 4" tig, I have 2 from different people, both 3-4" and one £80 and other £130. Just look that their feeding well and they will grow like weeds. Mine doubled in size in 3 weeks on feeders. Markings will really change as they grow so dont worry if they look abit plain. If I was you, I would leave them in the shop for a few weeks to grow then try and get for better price. Hope this helps