2 meter data cable to rgb LED
The datasheet for the WS2811 driver shows that the IC itself buffers the data sent to the next LED position. They do show 33 Ohm series resistors for impedance matching so make sure you have those. 

The most important aspect of selecting a wire size is to be ultra conservative about voltage drops down your wire. Here is a chart that shows the reduction in current for 'power' applications (overkill here but it serves to make the point on size selection). 

You might expect from the maximum rating of the WS2811 that you have 3 * 18.5 mA maximum (white) per device, so for 40 drivers you have a maximum current requirement of approximately 2 Amps at your source end. From this current requirement and the chart you could reasonably expect to select a wire gauge size between 26 AWG and 18 AWG depending on how conservative you feel.
A very common size for 'hookup wire' is 22 AWG, and since the current requirement reduces as you move down the string of LEDS, this might be quite adequate for your 2 M run of cable.