Wrt size of the raster, right click on the input raster, Properties->Source->Columns and Rows. This will tell you how many pixels are to be processed.
In the case of a MosaicDataset it is not that simple, easiest way to do it is to use one of SA's less complex tools to create an output from which to determine the number of pixels that'll be processed. You could simply use the Int tool and then look at the number of columns and rows of the output raster created by Int.
Or in your case you can look at the properties of your FlowDirection raster, and, how long did it take to get created?
You also need to state what version of ArcGIS you are using.
-Steve