Hi,
Adding to Gautam's and Maciej's answers, the 6 inches are a recommendation from TI and not part of the IEEE1149 standard. This is empirically determined to meet the JTAG data rates that correspond up to 50MHz of unbuffered TCLK without compromising data integrity - check section 6 of this page. There it also says that speeds would increase to 100MHz (with probably a reduction in the JTAG length), but that references a technology that was discontinued (high speed RTDX).
Keep in mind the distance is between buffers, and in the case of an emulator it is measured between the (edit) output drivers inside the emulator board connector and the device pins (not the board connector), therefore several emulators have cables shorter than 6 inches to compensate any length in PCB traces.
Hope this helps,
Rafael