if you delineate the flooded area with the band maths (e.g. pixel thresholding) you can as well create a mask in the Mask Manager using the same experession and then display the size of the mask as suggested here: Counting number of white pixels
if you delineate the flooded area with the band maths (e.g. pixel thresholding) you can as well create a mask in the Mask Manager using the same experession and then display the size of the mask as suggested here: Counting number of white pixels