In the Precision menu is an option to turn "Measure" on in your drawing. Measure applies live, ruler-like measurements to any line, shape or object you draw on the canvas. These measurements correspond with the current scale set to your drawing. If you're drawing a plan to a certain scale, for example, Measure will display measurements according to this scale.
To turn on Measure, open the Precision menu, then toggle Measure to "On". With Measure active, you have two ways to measure accurate distances in Concepts - via 100% Line Smoothing, or via the Line guide.
Measure + Line Smoothing
Set your current tool to 100% Line Smoothing to draw instant straight lines (use the smoothing slider in the inner ring of the tool wheel). As you draw, the line will display a measurement tag so you can see exactly how "far" you are drawing (based on the scale set for your drawing in Settings -> Drawing Scale).
As 100% Line Smoothing calculates the shortest length of your final line from start point to end point, the measurement will reflect an accurate final line length. This means you can draw forward and the measurement will add length, or backward and the measurement will subtract length.
To resize the line to an exact, custom length, select the line, then tap+hold the measurement tag (or the Length field (L:x) up in the status bar) and enter your custom length. The line will adjust to the new length. You can tap a measurement tag to "stick" it to your canvas and display the measurement.
Measure + Line Guide
The Line Guide in the Precision menu is a traceable ruler, allowing you to see and set measurements within the Guide handles. Measurements will display according to the scale you have set in Settings > Drawing Scale and Units for your drawing. Note that the Line Guide only measures additively, so if you trace the length back and forth on the guide, it will continue to add length (as your total line length is getting longer as you draw).
Open the Precision menu and tap Guide to activate the Shape Guides feature. Tap the Guide type label to bring up your Guide options, and tap the Line Guide. This activates the Guide UI on the canvas which lets you interact or draw with the Guide.
To set an exact length with the Line Guide, tap+hold the measurement tag next to the guide to bring up the keyboard. Enter your value, and the guide metrics - the length between the handles - will adjust to the length you entered.
From here, you can tap the measurement tag and it will place a floating measurement on the canvas.
You can opt to draw a line with the guide, as well. Before drawing, we recommend you double-tap the crosshairs at the center of the guide to constrain your drawing to between the handles, or you will easily draw past the length you've entered. We also recommend you lock the scale in the menu at the bottom of the canvas - it should say "Scale Off". This will keep you from changing the stroke or guide length when moving it around the canvas.
For more on using the various Shape Guides, entering exact angles and measurements, see the Precision guide in the manual here.