Can I be diagnosed with a micropenis based on my girth?

From your description it sounds like you do have a micropenis. Treatment would depend on many different factors. You may have a good sized penis but most of it could be buried inside the skin and fat.

Healthcare providers determine micropenis size by the stretched penile length (SPL). The proper way to establish the SPL is to gently stretch the penis, hold it close to the body and measure it from tip to base of the penis. Providers diagnose micropenis if the length is less than 2.5 standard deviations below the average. For example the average stretched penile length (SPL) for adult males is 5.25 inches. An SPL of 3.67 inches or less indicates an adult micropenis.
But what appears to be a small penis may not technically qualify as a micropenis. It may instead be a condition known as buried penis. Another condition that may be mistaken for micropenis is webbed penis, also known as an “inconspicuous penis.” With a webbed penis, skin from the scrotum is attached unusually high on the shaft of the penis. The result is that the penis itself looks smaller than normal because just the tip and some of the shaft is visible.

To diagnose micropenis, it should be 9 cm below the length of the penis. It varies according to height and weight. If you report your penis length, height and weight, we will inform you.