To determine Point Density the cell edge length can be set to whatever you want it to be set to for the purpose of computing the density, the density range is what is important. You’re looking for the number of points per square units. The cell edge length simply says take samples within a n by n grid. But the density itself is independent of that sample size.
For instance let’s say you have a cell edge length set to 10 meters, which means it is taking samples using a 10 m x 10 m grid. If you set the point density band to be 1.5 pts/square meters then it really means that you are looking for 1.5 pts/square meters and is completely independent of the sample size.
There are times where a specific cell size is required, for instance USGS specifications concerning spatial distribution or low confidence areas. In that instance you are looking for a certain value within the specified grid.