In a medium bowl mix chicken, mayonnaise, sour cream, and tarragon until well combined. Season with salt and pepper, to taste. Fold in grapes and pecans. Chill for at least one hour. Check seasoning again and adjust, if desired. The chicken salad can be made at least 24 hours in advance. Scoop chicken salad onto a slice of bread. Top with another slice of bread. Cut off crusts, then cut sandwich into desired shape (rectangles, triangles, or rounds).
Quantity of sandwiches will vary according to shape of sandwiches.