Depends on the size of the aquarium. If you have like a super massive (just for conversation) 50,000 gallon or more, then they'll get bigger than in the wild. But on average I don't think they do. I have seen a couple 4 footers but I mostly see 30-40 inches. I'm no fish keeping expert but IMO one (provided an incredible filter) rtc will do fine in a 8' x 4' x 2.5' pond because they usually don't reach their full potential unless there given extreme tanks. My LFS kept one in a 480 for 13-14 years and it only got 3.5 feet long. Sorry I'm not sure what the metric conversions for these are.
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In my opinion this is the minimum and you can only keep it alone because it will eat every other fish..