In a Fischer projection, each cross represents a carbon position: horizontal chemical bonds point toward the viewer and vertical chemical bonds point away. For the open-chain aldehyde sugars illustrated here, place at the top and at the bottom. This is a convention for stereochemistry, not a measured molecular shape. See Moss, IUPAC Recommendations 1996: Fischer projection.
In these Fischer projections, D specifies that the hydroxy group at the lowest stereocenter lies on the right: C4 for ribose and deoxyribose, C5 for glucose. D does not specify every other stereocenter, and it does not specify the direction of rotation of polarized light. See McMurry: the meaning of D and L.
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A Fischer projection is a two-dimensional representation of a three-dimensional organic molecule, specifically used for depicting the stereochemistry of carbohydrates and amino acids. It was developed by Emil Fischer in the early 20th century. In a Fischer projection, the molecule is represented as a vertical and horizontal arrangement of bonds. The vertical lines represent bonds that project away from the viewer (into the plane of the paper), while the horizontal lines represent bonds that project towards the viewer (out of the plane of the paper).