Your use of the term "dimensions" makes me think you're used to engineering design programs like AutoCAD, where you can add measurements like this:
QGIS doesn't have quite the same feature. It also doesn't use the term "dimensions." So if you tried to search for how to do this, you probably had very little success. Here are some ways you can add dimensional information to your map (relevant search terms are in bold):
You can add a scalebar to the map canvas or print composer/layout.
- to add a scalebar to the map canvas: View menu > Decorations > Scale bar
- to add a scalebar to the print composer: Add Item menu > Add Scale Bar
- The map canvas is what you're calling the "editing/drawing space"
- "Print composer" vs. "print layout" - This is where you design a map for printing or export. It was called the print composer in QGIS 2, and in QGIS 3 it's called the print layout. The terms are frequently used interchangeably, out of habit and because they're basically the same feature. I have no idea why the name was changed.
You can also label line features with their length using the
$length expression as a label. In the example below, notice that the line is labeled with its entire length; each segment is not labeled separately:
You need a separate line feature for every dimension you want to display.
- To show measurements of the edges of a polygon, use the
Polygons to lines tool to convert each polygon into a line. Use the
Explode lines tool to split the lines into separate segments.
Also see the several excellent answers to this question: QGIS How to label individual sides of polygon. The answer by she_weeds has a good walkthrough for using a geometry generator style to display dimension labels in a CAD-like style.