The more used app is the Image j of NIH. It is free and are available several tutorials like; usually relative area and stomatal index are suitable informations https://www.youtube.com/watch?v=k4yzAscUD1g

 I think you can take a set of known measures to determine a conversion coefficient and then create a line shapefile to draw and store in the shapefile dbf table the measures.

Just out of curiosity... Do you have a particular reason for using Arc View 3.0?

It is a quite old software and have been 'out of service' for most of a decade. The same company (ESRI) have newer versions, that are more advanced in most any ways. There are also free GIS software (QGIS, Grass, etc.) that will do most anything you could do in ArcView 3.x

We do not use even ArcView 3.2. ArcView 3.0 is rare artifact. 

Please use Qgis

Dears

Thanks for all attentions and responses.

We only want to count the stomata, stomatal clusters in plant (Melilotus alba) leaf and determine  stomatal distances. In this way we can use all methods. Please send me newer and simple method, if any.

Regards

Alireza