I had this question today asking how you set up the Takeoff process to report the lengths of objects like Silt Fence for a project. The key here is
Create your Site Improvement for Silt Fence as a Linear Site Improvement in the MSI Manager
Make sure that the layer that the Silt Fence is drawn on is categorized as something other than Unused or Unknown i.e. place it in Other category
Go to the Takeoff - Categorize Layers - Other Tab, locate the layer, click the check box next to it and then at the base of the dialog, select the “Associate with a Site Improvement” Button. Select the Site Improvement that you defined in the MSI Manager.
Now you can run the Takeoff Report and check the Site Improvement Quantities check box (Original Takeoff Report Only - these are not supported in the Custom Takeoff Report) - and run the report. You should now find the Silt Fence Quantity in the Report as needed.
The Video shows you how