Default value
32 (width of default icon; depends on world.icon_size)
This var defines how fast, in pixels, an atom will move by default. If you use lower values of step_size for most items in your world, you may want to consider raising world.fps (at higher performance cost).
When Move() is called by a step or walk, or by the built-in client movement verbs, a change of step_size is applied to step_x and/or step_y. Any movement within the speed of step_size, or up to one tile in distance (whichever is greater), is considered a slide and may partially succeed if an obstacle is bumped before reaching the final position.