You must be using projected data and the cell size must be in the same units. For example, if you are using a UTM projection, your coordinates will normally be in meters, so specify a cell size in meters. I suspect that you are using unprojected data...aka data in decimal degrees
I'm not sure I understand your problem. This is how I understand it: you are trying to run an interpolation tool like IDW. There is an optional parameter for cell size, in which you can select an existing raster to use its cell size. Since your output raster doesn't exist (yet), you can't select it.
