Areas return the most values when used, Area, Length, Volume and Wall both gross and net values, along several other values like Outer Dimensions.
For rectangular objects that you can use diagonal placement for areas when placing them on your blueprint. Start by clicking in the top left of your room and then right click or double click in the bottom right. You will see a rectangular box with a dashed line diagonally when this mode is active.
Rooms or shapes that have several corners or breakpoints can be measured by clicking at each corner and continuing around the lines. When you see the object taking the shape you want you can right click or double click on the last point to create the area. If you overshoot a small targeting square will appear on the starting point and clicking within that point will also complete the placement.
Change and Adjust
To adjust your measured area you can easily drag the small squares. You can add or delete points on your area by using hotkeys
ALT + Click to add
ALT + SHIFT + click to delete
You can press and hold shift while placing lines to lock them in 45-degree angles from the previous
You can use Reductions to remove parts from Areas. Read more about Reductions