First, width. The frame should be about as wide as your face at the temples. Wider looks borrowed; narrower pinches.
Second, the brow line. The top of the frame should follow your eyebrows or sit just below them.
Third, the bridge. If the frame slides, the bridge is too wide. If it leaves marks, too narrow. Adjustable pads forgive more than moulded acetate.
Get these three right and almost any shape will suit you.