Trace Dimensions limitation for JTAG signals
I would not be worried about loss, because you are not transmitting any power.  For a simple implementation, the trace length will dictate the signal propagation time on the bus, and therefore the maximum frequency you can run the bus at.

The trace impedance is also not critical:  As long as you maintain the impedance consistently, you will be able to deal with reflections.  If you only connect a single JTAG target, you can get away with series termination of your signals.  For a typical 50&Omega; impedance trace, add a ~30&Omega; resistor in series with the driving pin (close to the JTAG adapter for TDI, TMS, and TCK; close to the JTAG TAP for TDO).  Of course the exact numbers depend on the ICs you are using and their driving abilities.

The longer your signal traces are, the more capacitance they will present to the driving buffer.  A higher capacitance will reduce the maximum speed you can attain over the link.