Measuring the Pixel Size of Objects inside Images

I am attempting to write a program that will compute a change in size of an object from one image to another image of the same object. Example being the two images of a simple square and a bigger simple square. I know that MATLAB has a toolbox that has a simple measurement tool but I'm trying to get the code to were I can simply link two images into the code have it compute a specific value. I have the code to the point that the code will read in the image find the object and impose the pixel size of the entire image but I am having troubles getting the code to compute the pixel size of a specific object inside the image. I'm sure there is MATLAB function I have overlooked that would make this easier and was wondering if anyone could point point me in the right direction. Thanks for any help or advice in advance!

clear all;
clc;

I1 = imread('http://i46.tinypic.com/xoqzya.jpg'); %first original image
I_1gs = rgb2gray(I1); %convert RGB img to grey scale img
I_1mat = im2uint8(edge(I_1gs,'canny',0)); %find the edges of the image and convert to matrix form

D1 = bwdist(I_1mat,'euclidean'); %distance transform
D1_1 = mat2gray(D1);
figure
subimage(D1_1) %convert from matrix to grey sca

I2 = imread('http://i50.tinypic.com/34nodfs.png'); %second original image
I_2gs = rgb2gray(I2); %convert RGB img to grey scale img
I_2mat = im2uint8(edge(I_2gs,'canny',0)); %find the edges of the image and convert to matrix form

D2 = bwdist(I_2mat,'euclidean'); %distance transform
D2_1 = mat2gray(D2);
figure
subimage(D2_1)

D3 = D1 - D2;
D3_1 = mat2gray(D3);

for n = 1:j1
    for m = 1:k1
        if D3_1(n,m) == 1
            D3_1(n,m) = 0;
        end
    end
end

figure
subimage(mat2gray(D3_1)) %image of the difference between image one and two