On a project, click on the Parcels tab to expand and view the full parcels list
In the top-right corner of the parcels list, click on the 3 vertical dot icon
From the dropdown, select Columns
In the columns sidebar window, type in 'Buildable' to filter the list of columns to just those related to the buildable area.
Click the checkbox next to Buildable Area (Acres) to view the column in the list view
Another helpful column is the 'Buildable Area (Relative) column. This will display a ratio of the buildable acreage vs. the original lot size (e.g. a Buildable Area (Relative) = .64 means that parcel was 64% buildable)
Once this box has been checked, the Buildable Area will show up as an attribute column in the Parcel List:
Check out how to sort/filter the parcel list based on the buildable area (acres) attribute (or any parcel attribute).