As others mentioned, you can have the object's dimensions in the side bar. But understand it's only the objects panel from a bounding box. In your case, the dimensions are calculated from a box kind of like this :
You can use the MeasureIt addon. AFAIK it's included in Blender.
If you click the little + sign on the right side of the 3d viewport, it will bring up a bar. You can also press N to bring it up. You can see the dimensions in the top of that panel. It is labeled with "Dimensions"
I don't have the score to make a comment, but I wanted to include in @rob's comment that the information is at the top and below the position information.