Mapinfo Pro: .grid file area

Try drawing a polygon the same size and shape as the raster (cosmetic layer should be fine) and then interrogate the polygon. For a more accurate answer you could draw a larger polygon and clip it using the raster.

If you are using MapInfo Pro Advanced, the Statitics tool reports number of valid cells. Multiplying this number by the Area of one cell should give the exact area of the valid cells of Raster. Cellsize of a Raster can be seen in the RasterInfo tool. So it's like:- (Cell size X) x (Cell size Y) x (number of valid Cells).

You can use the Polygonise tool under Raster Operations.

This will let you create vector polygons from for example the valid data in the raster grid.

Note that this does require MapInfo Pro Advanced.