From Basin Wiki

Many skill descriptions (notably those for a Necromancer's Curses and a Paladin's Defensive and Offensive Auras) specify radius in yards. However, radius is specified in sub-tiles in the game code; each sub-tile is a 2/3 yard by 2/3 yard square (i.e. 2 feet by 2 feet). Below is a diagram displaying both sub-tiles and radius in yards:

The radius of many skills increases with skill level, but skills with an invariable radius have been labelled:

Skill	Radius
	Sub-tiles	Yards
Terror	4	2 2/3
Decrepify	6	4
Iron Maiden	7	4 2/3
Attract	9	6
Redemption	16	10 2/3
Inner Sight Slow Missiles Conviction	20	13 1/3
Cloak of Shadows	30	20

Cloak of Shadows isn't labelled since its radius encompasses almost the entire visible area, with the exception of a tiny portion in the top left and right corners.

Sub-tiles

Diablo II uses an isometric projection which divides the horizontal playing surface into rhomboid tiles, each of which is composed of 5 x 5 sub-tiles. Below is an image of an individual sub-tile, magnified so that you can see the individual pixels and how the sub-tiles fit together:

Visible Area

Since a sub-tile is a 2/3 yard by 2/3 yard square (2 feet by 2 feet), this means the hypotenuse of the right-angled triangles created by bisecting that square from opposite corners is the square root of 8 (2² + 2²) in feet i.e. approximately 2.83 feet. Horizontally this distance equals 32 pixels, but vertically this distance equals just 16 pixels, resulting in the following:

Resolution	Visible Area
	Pixels	Yards
640 x 480	640 x 433	18.86 x 25.51
800 x 600	800 x 553	23.57 x 32.59

If it seems counter-intuitive that the vertical distance in yards should be greater than the horizontal distance (since it's fewer pixels in length), take another look at the diagrams above; as an example, the radius of Inner Sight, Slow Missiles and Conviction is within the top and bottom limits of the visible area, but extends beyond these limits to the left and right.