I have been trying to figure out the best infant mattress. The crib we purchased is the Kendall from Pottery Barn. They told us at Babies R Us that they are all the standard size like our crib but they all have different dimensions. If they have different dimension how are they standard? I need help and recommendations. Also the crib says not to exceed 6 inches thick for the mattress but two out of the three most popular were over 6 inches and I read on Consumer Reports it should be between 4-6 inches thick. Please help!