15"x23" from outer edge of silk border. Painting on fabric. Unfortunately like many, it has been glued to masonite or something similar. Some water damage drip down the center, and some scuffs and scratches. Written on the brown paper frame backing " Purchased by Mr Avigdor in Capri Italy from German countess" (Mona Von Bismarck)? I know that's not good provenance, but I figured I'd include it.
Only what is written on the back, for what it's worth. " Property of Mildred D (unreadable surname) purchased by Mr Avigdor in Capri, from a German Countess" Was found in the attic of a old house in New Hampshire.
Your thangka is indeed made in Tibet and its made late 19th century so its an antique one. Its a Buddhist painting of Avalokiteshvara or a representation of the Deity of compassion. He is one of the three protectors Manjushri, Avalokiteshvara, and Vajrapani. This is a quite common thangka and the painting is not above average to be honest. Still there is demand for antique thangkas; Buddhism is quite popular in the west the last two decades and that reflects on demand for original items from the main countries and especially Tibet and Nepal. If you would like to sell I recommend an auction house with a prescription to an online platform like Invaluable, live-auctioneers or other. That way you are sure there is a good international audience for it.