How to detect number of specific object in an image and it's diameter(size)? I have two images of IRON RODS (i.e only two images) and I want to detect how many rods are there in each image and get the diameter of each rod in a image?

I don't know where to start as there are lot of articles regarding object detection and it's hard to find one for my use-case.

If you can help me with a abstract view on how to model such requirement? Please point me a right direction. 

Doubt:

1) How can I train with two images and then how to count numbers of rods are there in each image and retrieve it's diameter?
Well to answer your last question, you cannot train with the images that you want to test your algorithm with, that is a paradox...

You will need more than just two images if you want to build a robust model that can detect each rod AND predict their diameter. Those are two different problems, and you might want to split your model in two branches: one to detect each rod, the other to predict its diameter as a regression problem.

If you can obtain a larger dataset with ground truth, then you can proceed to train your model. I would tend to say that using an object detector CNN would be overkill, but if you need to predict the diameter of each rod in the image, it might be necessary. So maybe look into TinyYOLO for the detection part, and then you could use the predict bounding box of each rod in the image for a small perceptron to infer the diameter.