Not sure if the "X" moniker is quite justified here, considering it's a small sensor bridge camera. No mention of the actual size of the sensor though apart from it being a "backside-illuminated CMOS sensor"
Pentax X-5 | PhotographyBLOG
Pentax X-5 - Digital cameras - CNET Reviews
Pentax intros X-5 superzoom camera with 26x lens, tiltable LCD -- Engadget