Hi Sky Lee,
2590 is normally used in combination with CC25xx and CC254xx series.
The part has been tested with max output power of those devices (typically from 0.3 to 4.5 dBm), providing max output power of 14 dBm.
For information on CC2590, please visit http://www.ti.com/lit/ds/symlink/cc2590.pdf
Thanks,
TheDarkSide