Here is a chart with some suggested sizes:



And from Master Airscrew:

2-Stroke Engines

.049 - .051 = 5.5x4, 5.5x4.5, 6x3, 6x3.5, 6x4

.09 - .10 = 7x3, 7x4, 7x5, 7x6

.15 = 7x6, 8x3, 8x4, 8x5, 8x6, 8x7

.20 - .25 = 8x6, 8x7, 9x4, 9x5

.29 - .35 = 9x6, 9x7, 9x8, 9.5x6, 10x4, 10x5, 10x6

.40 = 9.5x6, 10x4, 10x5, 10x6, 10x7, 10x8, 10x9

.45 - .50 = 10x7, 10x8, 11x4, 11x5, 11x6, 11x7, 11x7.5

.60 = 11x5, 11x6, 11x7, 11x7.5, 11x8, 11x9, 11x10

.71 - .80 = 12x6, 12x8, 13x6, 13x8, 13x10, 14x8

.90 = 13x6, 13x8, 13x10, 14x6, 14x8

1.08 = 14x8, 15x8, 16x6

1.20 = 14x8, 15x8, 16x6

1.5 = 16x8, 16x10, 18x6, 18x8

1.8 = 18x8, 18x10, 20x6, 20x8

2.1 = 20x8, 20x10

2.7 - 3.5 = 22x8, 22x10, 22x12, 24x8, 24x10, 24x12

4-Stroke Engines

.20 - .25 = 9x4, 9x5, 9x6, 9x7

.40 = 11x6, 12x6

.60 = 11x8, 11x9, 12x6, 13x6

.90 = 12x8, 13x8, 14x6

2.10 = 14x8, 15x8, 15x10, 16x8