The Net trace length is displayed in the Nets table (located under the display window - see screen capture below).
HINT: The fastest way to find the Net Length is to use the command "Query Net" (menu: Info / Query Net), and select any object on the net. Query Net will highlight the entire net and also highlight the net within the Nets table.
Cool Feature #1: The Nets table is completely sort-able, so you can select the Length header and determine which nets are the longest, shortest, etc.
Cool Feature #2: Using the Nets Table you can Highlight Multiple Nets at once (use SHIFT and CTRL keys for multiple Nets selection).
Cool Feature #3: You can also "Right-Click" within the Nets table for more features, such as export the Nets to an Excel file and more.