Sprite dimensions:

Monster 1, 1240x136 px - Cell size 40x34 px

Monster 1 spikes, 48x16 px - Cell size 16x16 px

Monster 2, 688x230 px - Cell size 43x46 px

Monster 3, 976x158 px - Cell size 61x37 px

Monster 3 skull , 96x24 px - Cell size 24x24 px

Monster 4, 870x148 px - Cell size 58x37 px

Monster 5, 840x240 px - Cell size 60x48 px

Monster 5 web, 128x32 px - Cell size 32x32 px

Monster 5 brood, 396x125 px - Cell size 33x25 px

Monster 6, 1606x136 px - Cell size 73x34 px

Monster 7, 1444x236 px - Cell size 76x59 px

Monster 8, 950x92 px - Cell size 50x23 px

Monster 9, 700x110 px - Cell size 35x55 px

Monster 10, 720x100 px - Cell size 48x25 px

Monster 11, 770x144 px - Cell size 70x36 px

Monster 12, 506x172 px - Cell size 46x43 px

Monster 13, 1056x72 px - Cell size 66x36 px

Monster 14, 671x230 px - Cell size 61x46 px

Monster 15, 2400x186 px - Cell size 75x62 px

Monster 16, 728x200 px - Cell size 52x50 px

Monster 16 bullet, 102x11 px - Cell size 17x11 px

Monster 17, 387x102 px - Cell size 43x34 px

Monster 18, 1771x129 px - Cell size 77x43 px

Monster 18 attack fx, 880x50 px - Cell size 88x25 px

Monster 19, 490x62 px - Cell size 49x31 px

Monster 20, 804x126 px - Cell size 67x42 px