use region properties of the image like :

s = regionprops(YourImage == 0,'Area','PixelIdxList')

count the pixels in the segmented region by label value

sum ( image(:) == segment_label)

or by black value itself

sum (image(:) == 0)

You are supposed to use some morphological processing methods. Matlab provides a numer of functions like 'regionprops' which can help you measure connected pixels in your image. In this case, you can use 'imcomplement (.) '+'regionprops(.)'