Depends on what you want to measure: area, diameter or radius?
In analyze > Set measurement, select Area and Feret's diameter (optional: perimeter).
Calibrate your image (e.g. with an object of known size/area), select oval measurement and draw circle over the plaque. Measure (CTRL+M).
When plaques are perfectly round, you can check the results by using formulas for perimeter = 2x PI x radius, and area = PI x radius^2.
Does this help?
Also, I want to know is what is the best parameter for measuring the plaque whether I should go for the diameter of the area.
I don't know the other details of your research (is it related to viruses?). For antibiotics (disk diffusion susceptibility test) it is customary to use diameter (zone of inhibition). But when plaques are more oval or irregularly shaped, the area would be a more accurate measure.
First, make sure the plaque is clearly visible and free of any obstructions. . Position the measuring tool over one edge of the plaque, making sure it lines up with the edge of the plaque. Using your eye or a magnifying glass, make sure you get an accurate look at the edges of the plaque. Record the starting position aligned with the plaque edge on the measurement tool. Continue moving the measurement tool until the other edge of the plaque is reached. Record the end position aligned with the plaque edge on the measurement tool. Finally, use the scale or markings on the measuring tool to determine the distance between the start and end points, which will be the diameter of the plaque.
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