To find the area you may need to segment your image. If you just want to define the area's vertices somehow, like with roipoly() or imfreehand(), then you can get the area with polyarea(). However if the area is defined by something like intensity, texture, or whatever, then you need to segment the image. Segmentation means undergoing a series of steps such as filtering or morphology or something until you eventually get to an image that can be thresholded to form a binary image (your segmented image). With that binary image you can then label it with bwlabel() or bwconncomp() and then call regionprops(). Or you can call bwarea() for a different definition of area, weighted by how the boundary winds around, whereas regionprops() gives area as a simple count of pixels in the blobs.