This is not a tutorial on architectural or engineering dimensioning (Coming soon). This article is how to ascertain the size of an object and how to use the Measure tool.
At first glance, the easiest way to ascertain the size of an object is to select it and then look at the Dimensions in the Item tab of the “N” menu. This is a perfectly valid method – sometimes, but not always…
We’ll start with a 2m x 2m cube which has one face inset and then extruded out by 1.5m :
A look at the Item tab of the “N” menu shows:
Now, lets make the bounding box visible.
Select the object and click the Object Properties tab 
of the Properties Editor panel. Expand Viewport Display and click the checkbox Bounds :
See here for a discussion on bounding boxes
The problems start if we rotate the object and apply the rotation 
like this:
But if we look at the Dimensions panel, we see that all three dimension values have changed. That’s crazy! We didn’t adjust the size of the object!?
No, not crazy! what is happening is that the Dimensions panel shows the dimensions of the Bounding Box, and not the actual dimensions of the object’s length, breadth and height.
Measure using the Measure Tool “Layer”
In blender, the Measure tool has a rather non-intuitive way of working and, when compared to CAD programs, lacks many features. However, it is good enough to measure the length of edges and the distance between vertices accurately in 3D space. It is activated by clicking the Measure icon on the vertical toolbar, which then shows a mini-help list:
The key to using the measure tool easily and effectively is to press the 
key when grabbing the corner of the object, to accurately snap to it.
Once the Measure layer is activated by clicking the measure icon, move the mouse cursor close to a corner and hold the key down. As soon as you are close enough for Blender to decide which corner you want, a small circle appears over that corner: 
Left-Click and Drag 
to anchor the one end of the measure line. While still dragging and holding the key down, the small circle starts dragging a line.
Move across to the other corner and Left-click
. The dimension appears. Try adding a few other dimensions and you will notice that as you drag, the circle stops at key points like the middle of a edge, for example:
The dimensions will remain visible while the Measure tool 
is selected. As soon as another tool from the toolbar is selected, the dimensions disappear. Clicking on the Measure tool again brings them back.
To delete a dimension, click the text which changes it into an angular dimension. Then press the 
key to delete it
Do you have to have the bounding box enabled in order to use the measure tool? No, but the bounding box helps to clarify the details of the object.
For more info, here is a link to the official documentation of the measure tool
