Using the Raster to Polygon tool, convert the integer type raster to a polygon feature class.
1. Select ArcToolbox > Conversion Tools > From Raster > Raster to Polygon from the menu.
2. In the Raster to Polygon window, in the Input raster field, enter the raster produced in Step 2.
To discover the region, right-click the output raster you just made, select Properties, then the Symbology tab, and take note of the Count for the pixels that are shown. Note the cell size and linear units by clicking the Source tab.
The "area" function of R's "raster package" returns a vector of the raster object's cell sizes (in km2), which vary from north to south. To compute the size of the overall area, sum together all cell sizes or multiply the median cell size by the number of cells.