Hi Bob, hi folks.
To add to all the very good answers, to draw a line, use the LINE TOOL, not the TAPE MEASURE TOOL. The Tape Measure Tool can be used AFTER drawing an object, line or whatever to modify it but, why correct an error if you can avoid it.
You need to input the correct line length WHEN creating the line, not AFTERWARD.
Of course, any of the 2 endpoints of a line can be moved, thus changing the length but, again, this is a waste of time.
The Measurement Window is normally at the right end of the Status Bar. It can me moved anywhere but, personally, I prefer to have it docked at the right end of the Satus Bar so I know every time where it is.
The Status Bar is at the bottom of the Model Window. If this Status Bar is not visible, try maximizing your Model Window.
The Status Bar is very important, especially when you are learning SketchUp. Observe it after each of your action, it tells you a lot of helpful hints.
For example, to draw your 16’ 3" line, you do the following:
1 - Choose the Line Tool (the icon that looks like a slanted pencil). The status bar shall show this text : “Select start point”. The Measurement Box shall be preceeded by the word “Length”.
2 - Click the Left Mouse Button (LMB) to place the first endpoint of the line. The Status Bar shall sown this text: “Select end point or enter value”. The Measurement Window shall still be preceeded by the word “Length”.
3 - Move the mouse somewhere else and observe the Measurement Window. You shall see that the number in it changes as you move the mouse, giving you a feedback of what the actual length is. Don’t bother to get it close to 16 feet or whatever, the important thing here is its orientation. If you draw the line with a correct orientation, you save a non necessary step or reorienting the line.
4 - Let go of the mouse and then type 16’3". You can put a space between the ’ and the 3 but SketchUp is good at interpreting the characters that you type in the Measurement Window. Like other wrote, do not click in the Measurement Window, it becomes active automatically as soon as you begin moving the mouse, thus is awaiting your input.
5 - Press the Return Key to tell SketchUp that you have finished entering the required length. The line length shall change to reflect the new value of 16’ 3". If you don’t choose immediately another Tool, you can retype any new value for your line and see it changes each time you press the Return Key.
Now, something important, if, for any reason, you have zoomed the model out in a significant way, a 16’ 3" line may appear as a tiny speck or may even not be visible at all. Similarly, if zoomed in a lot, the line may not fit completely in the Model Window.