Binarize the image, then use region properties to compute the area (number of pixels). Once you know the number of pixels, you can get the area by multiplying by the area of one pixel.
Im = imread('29_predict.bmp');
mask = Im>188; % binarize the image
area = regionprops(mask, 'Area');
areaReal = area.Area*0.01^2; % real-world area
If you have a 3d image (stack of 2d binary images), you can also call regionprops to compute the number of voxels. To get the real-world volume, multiply the number of voxels by the volume of one voxel.