You can try to automate the measure by using Analyze>Analyze Particles... after thresholding the image. You may need (or may not) to previously convert to 8bit first. You can obtain area, perimeter, etc for each particle. Of course, as Mohamed metions image need to be calibrate and for that you need to know the scale.
The magnification is ok. You just need better resolution. Your pixel size should be several times smaller that the diferences in diameter that you expect between control and exp groups-