6’1 on a 21″ tt. One of my BMX riding buddies is 6’2 on a 21″ tt. We ride at indoor and outdoor parks, and BMX tracks. Long tt or not a 20″ wheeled BMX is going to feel small, you’ll be fine on a 21″. If the price is too good to miss, then just get it asap, you’ll have a blast, they’re amazing fun 😀
The only time it feels twitchy is when I’ve been out for a long XC ride on the 29er on a Sunday, then hit the BMX track on a Monday – what a contrast, normally results in a few wobbly moments and landings on the first lap but you very quickly adjust