At 6 feet, you can probably fit on most medium and large bikes out there. The big difference between your Rampant and many DH bikes is the seat tube angle. A typical DH bike wil have a seat tube angle in 55-65 degree range while your rampant is probably 71-73 degrees. When you measure effective top tube length you go all the way from the center of the headtube to the center of the extension of the seat tube along a line parallel to the ground (horizontal line).
On the other hand, the measurement that really matters on a DH bike is what is called "reach": this is the horizontal distance from the bottom bracket to the center to the headtube. This is a measure of the size of the cockpit when you are standing and not seated.
Banshee lists reach and stack measurements for their bikes, so you can look up both the Rampant and the Legend and compare the sizes. You can also use a geometry calculator such as this one:
http://bikegeo.muha.cc/
to calculate reach and stack for other frames you are considering.
In the end for you the choice between medium and large will probably come down to riding style and the kind of terrain you ride most frequently. Large frames: longer wheelbase, more bike in front of you, more stability at speed. Medium frames: more weight on the front end, more agile in the tight and twisty and slow speed manouvering.
On the other hand, the measurement that really matters on a DH bike is what is called "reach": this is the horizontal distance from the bottom bracket to the center to the headtube. This is a measure of the size of the cockpit when you are standing and not seated.
Banshee lists reach and stack measurements for their bikes, so you can look up both the Rampant and the Legend and compare the sizes. You can also use a geometry calculator such as this one:
http://bikegeo.muha.cc/
to calculate reach and stack for other frames you are considering.
In the end for you the choice between medium and large will probably come down to riding style and the kind of terrain you ride most frequently. Large frames: longer wheelbase, more bike in front of you, more stability at speed. Medium frames: more weight on the front end, more agile in the tight and twisty and slow speed manouvering.
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