I want to know which one of image processing algorithms is the best to get object size information ? (E.g. I have many objects in our image and I need to know which one is bigger or smaller and use this information to compare).
I think that if you could detect the object in the mage ( by using object detection algorithms SURF , SIFT or Brief ) so you can in the next step using a simple formula which is related to determine the distance between the edges for that object.
Firstly, detect the objects in binary image; in Matlab, you can use 'bwlabel'. After that, you can use region descriptors formula like 'Area of region'.
Convert the image into binary representation. If you have color image, convert the image into grayscale image and then convert it into binary image using thresholding method. Area of each object can be easily determined from the binary image. If you are using MATLAB, you may use 'regionprops' to know the area.
There is not single algorithm with the capacity to obtain the size. First, the object should be detected, Then, the object should be insolated. Finaly, the size of the object can be evaluated