If making a tart, one inch pieces are excellent. If making small size tarts, perhaps cut them a little smaller, So you get more fruit to fit in.
Raw rhubarb is quite sturdy, but as it cooks, it cooks down and gets very soft and diminishes and can disintegrate to mush if overcooked. So no, I do not recommend you use a food processor or cut the pieces too small for baking. If making muffins or quick bread, you may want to cut the pieces a little less than an inch.