I think there's only one such product SparkFun carries;http://www.sparkfun.com/products/10259
The datasheet doesn't mention any maximum length, but there undoubtedly is one; I got 5 meters on a roll once (no need to combine 1 meter segments explicitly and it's a heck of a lot cheaper). At 60mA per 3 LED group, 60 LEDs per meter, that's 20 groups per meter, so 1.2A per meter. The 5 meter I had was thus 6A and needed a fairly beefy power supply to run. When it ran, it (edit: the strip) got fairly warm to the touch (they do suggest you actually stick these to some manner of profile if the mounting surface is not particularly thermally conductive) but that's more likely to be the LEDs than crazy things going on in the strip's wiring. More importantly, the losses along the 5 meter length of the strip were so great that what was a cool white at the connecting end of the strip, became pink at the other end. This was also noted with the little paper I got with mine: connect both ends.
So even if you're not too worried about the amps through the strip, you'll likely have to re-connect every once in a while anyway. For your 6 meters, maybe every 2?