Simply put, the artist first sets a vantage point where the subject and the drawing surface appear to be the same size.
Then, using a variety of measuring tools (which can include strings, sticks, mirrors, levels, and plumb-lines) the artist draws the subject so that, when viewed from the set vantage point, the drawing and the subject have exactly the same dimensions.
When properly done, sight-size drawing can result in extremely accurate and realistic drawings. It can also be used to draw the exact dimensions for a subject in preparation for a painting.
Professional painters will in time, develop an ‘eye’ that precludes the need for measuring devices and plumb-lines, but the observation method itself is not abandoned – instead it becomes second nature