Steps:
"I need to find the value of a coin in the image"
- Preprocessing (If required)
- Segmentation (ROI)
- Apply Sort of Morphological Operations (If Required)
- Apply the bounded Box
- Use OCR
"its diameter and area"
- Preprocessing (If required)
- Apply the CHT or use the Circular Object Detection Method Link
- You may get the detais in resultant structre
If you are using another approach to obtain circular objects to obtain area and diameter, please use the regionprops function.
https://in.mathworks.com/help/images/ref/regionprops.html