Recently, I am creating an ad to market my dealership service. I am looking to publish this ad at different social media platforms. In fact, I don't have much budget to hire professionals or graphic designers in this regard that's why I am managing everything myself.  

I am finalizing the idea of advertisement but looking for any online tool to measure the actual size of images that I am going to use in it. Also, want to measure PPI. My friend Linda just suggested me this online tool to measure the actual size of small objects. Do you guys think is it enough worthy? Or should I go with any other tool?

Please guide me in a serious manner.