Criteria for good image size:

1. even number (and divisible by 4)
   
2. largest prime number factor is small (<=7) to make FFT faster
   List of good numbers (<=2000):
   All: [16, 20, 24, 28, 32, 36, 40, 48, 56, 60, 64, 72, 80, 84, 96, 100, 108, 112, 120, 128, 140, 144, 160, 168, 180, 192, 196, 200, 216, 224, 240, 252, 256, 280, 288, 300, 320, 324, 336, 360, 384, 392, 400, 420, 432, 448, 480, 500, 504, 512, 540, 560, 576, 588, 600, 640, 648, 672, 700, 720, 756, 768, 784, 800, 840, 864, 896, 900, 960, 972, 980, 1000, 1008, 1024, 1080, 1120, 1152, 1176, 1200, 1260, 1280, 1296, 1344, 1372, 1400, 1440, 1500, 1512, 1536, 1568, 1600, 1620, 1680, 1728, 1764, 1792, 1800, 1920, 1944, 1960, 2000]
   2: [16, 32, 64, 128, 256, 512, 1024]
   3: [24, 36, 48, 72, 96, 108, 144, 192, 216, 288, 324, 384, 432, 576, 648, 768, 864, 972, 1152, 1296, 1536, 1728, 1944]
   5: [20, 40, 60, 80, 100, 120, 160, 180, 200, 240, 300, 320, 360, 400, 480, 500, 540, 600, 640, 720, 800, 900, 960, 1000, 1080, 1200, 1280, 1440, 1500, 1600, 1620, 1800, 1920, 2000]
   7: [28, 56, 84, 112, 140, 168, 196, 224, 252, 280, 336, 392, 420, 448, 504, 560, 588, 672, 700, 756, 784, 840, 896, 980, 1008, 1120, 1176, 1260, 1344, 1372, 1400, 1512, 1568, 1680, 1764, 1792, 1960]
   

Note: some of the box sizes (52 88 104 132 176 208 220 264 308 352 364 520 528 660 704 728 924 936 1020) included in the EMAN2 boxsize page have large prime number factors (>7) and should not be used.