Manzanilla Sherry glass
A smaller tulip-shaped glass called a catavino is used for sherry. Ideally, they should be 5-6 ounce glasses, but you should only fill them about half full (2-3 ounces of sherry), though this thristy caballero on horseback at the Feria de Sevilla has his catavino nearly full.
An alternative is a tulip-shaped 6-ounce Champagne glass will do nicely. The Champagne flute should be one that curves in at the top, not one that flares out, so that you can more fully enjoy the wonderful fragrant smells of sherry.