After converting a class (ROI) to a vector layer, the length of each polygon in each class is one of the attributes that is calculated. It is just the length of the perimeter of the polygon. I am not sure how that tells you the 'length in the direction of wind'. There is the Mensuration tool in the ENVI interface that you can use to draw a line and measure the length.

Thank you for the information, it seemed it was most probably the perimeter value (and not unidirectional length), thank you for the confirmation.

I am aware of the mensuration tool, however there are so many samples that it would be indeed painstaking in order to do that. Is there another way to measure a vector in a specific direction?

Sorry, I don't know of any tool that will automate this or calculate the distance across a polygon based on direction (assuming a node to node calculation). If it is not node to node, it would be even more complex to determine automatically.