At the first time Digimon Master Online was released, the different of 3/5, 4/5 , and 5/5 egg is very clear. Here was the list :
3/5 Egg has a size range at 82%-100%I give the example, like the picture below. First is 3/5 Drimogemon, and second is 4/5 Elecmon.
4/5 Egg has a size range at 90%-125%
5/5 Egg has a size range at 118%-130%
However, it becomes unpredictable now. Last time, my friend hatch 5/5 Gabumon Egg and it only has 86% size, but the evolution stage is opened until mega stage. Moreover, a 4/5 salamon can go from 117% to 126%. Pretty confusing.
To know more about size differences,visit "All About Size Differences"