...
The bitmap size is the number of points in the grid (ecCodes key "numberOfPoints").
When encoding, you can use the key "missingValue" to tell the library where data is missing.
...
When the library encounters a value equal to the missing value in the data array, it will set the bitmap entry to 0 for that grid point. For an example see grib_set_bitmap
When decoding, you can directly query the bitmap to discover missing data values. For an example see grib_iterator_bitmap
...