To get the best fitting jon jon for your little one: Measure from about one inch below the back of his neck (about where the garment will sit) down the center of his back under his diaper (where the snaps of the jon jon will be). Here are the measures:
0-6m 12 inches center back seam
6-12m 13 inches center back seam
1T 14.5 inches center back seam
2T 17 inches center back seam
3T 18.5 inches center back seam
4T 19.5 inches center back seam
For all other garments:
Size 6M fits 19 inch waist and approximate height 28 inches.
Size 1 fits 19.5 inch waist and approximate height 31 inches.
Size 2T fits 20 inch waist and approximate height 34.5 inches.
Size 3T fits 20.5 inch waist and approximate height 37.5 inches.
Size 4 fits 21 inch waist and approximate height 41 inches.
Size 5 fits 21.5 inch waist and approximate height 44 inches.
Size 6 fits 22 inch waist and approximate height 47 inches.
Size 7 fits 23 inch waist and approximate height 50 inches.
Size 8 fits 24 inch waist and approximate height 53 inches.