His lower right hand is in Varada mudra, boon bestowing pose. Strangely Siva is holding a mace in his lower left hand. So far have not come across any sculptures of Siva with mace as an attribute. Khatvanga, a skull mounted on a staff, is an attribute of Siva, especially in his Agora forms.
This may be a depiction of Harihara, syncretic version of Siva and Vishnu. Not likely though.
About 10 cm in height. This is likely from Orissa, well that is what the auction house said. The peaked crown corroborates the regional attribution. May date to the nineteenth century or a bit earlier.