If they don't overlap and have a pure white background, it's trivial. Just threshold and calculate the area(s). See my image segmentation tutorial http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862
If they overlap it's much tougher. For example if that small Vhai bottle was positioned right at the center of the larger bottle, then you may have to look at other characteristics, such as finding a cap and estimating the size of the cap. That would be a huge jump in complexity from a simple thresholding situation. Hopefully your homework assignment or small project doesn't want you to do something as difficult as finding overlapped and partially visible (obscured, occluded) bottles..
