spell range?

http://leagueoflegends.wikia.com/wiki/Range

Note that autoattack range is measured differently to ability range. Given a source unit's hitbox and a target unit's hitbox, range is calculated as follows: autoattack range from edge to edge; targeted skill range from center to center; skillshot range from edge to edge; and the range of self-centered area of effects are from the edge of the source (the larger champions don't benefit less from auras than smaller champions).

This means that:

• Darius' E range would be measured from edge to edge
• Ryze's W range would be measured from center to center

So, a portion of Ryze's hitbox would be within the range of Darius' pull if Ryze is within range to cast W.

If you want the edge-to-edge range of Ryze's W, you would have to subtract roughly half of the width of Ryze's hitbox and Darius' hitbox from the ability range.