- Do some reading.
Flatland bikes have come down in size, but the majority of pros riding them are in the 5'10" - 6' range. I'm only 5'8" so I go for the smaller ones. But, there are a few slightly longer ones and you can set your bars up a bit higher. Yet, they are definitely shorter than they used to be and most find this helps with the quickness and agility these bikes need.
GT makes one flatland bike, but it is not USA available. Not sure why.
GT went bankrupt a few years ago and took all its subsidiary companies with it - Robinson, Powerlite, and Dyno are gone. Another group bought the rights to the GT name, and kept only one pro rider from the GT team - Jamie Bestwick (good decision).
There are a lot more choices in flatland specific bikes, but you likely won't recognize any of the names.
The SINGLE BEST ENTRY LEVEL FLATLAND BIKE: DK Signal This would blow away a Haro Team Master from 1988.