I know it's a pain, but the best way is to measure yourself. Get a pair of calipers, and put a piece of masking tape on the inside of each measuring arm (to prevent scratching the scope). Zero the calipers, then measure the diameters of the objective and eye piece. Then look at the chart below, and pick the size directly smaller than your measured value.
www . butler-creek.com/productsImages/charts/flip_scope_chart.html