When the Auto Resize attribute is on, the boundaries (Size and Resolution) of a fluid container dynamically resize as the fluid's density increases or decreases. As the fluid density dissipates, the fluid boundary resizes to accommodate only positive fluid density. An Auto Resize Threshold attribute in the fluidShape node’s Auto Resize section lets you set a density threshold value that causes the fluid container to resize. The higher you set Auto Resize Threshold, the higher the fluid density at the container’s boundaries. When Auto Resize is on, the voxels in the container are kept square. See Keep Voxels Square.

Fluid Auto Resize also works with fluid containers that have animated fluid emitters. When you use an animated fluid emitter with Auto Resize on, the fluid container expands or shrinks in response to the position of the emitter. Turning on Resize To Emitter ensures that the fluid container is positioned at the fluid emitter's location at the start frame. See Resize To Emitter. For information about ways to control the boundaries of auto resized fluid containers, see Controlling auto resize boundaries.

Auto Resize works well with fluid effects that move quickly, such as a missile vapor trail or the rolling smoke of an explosion, as it keeps the fluid container relatively small. Turning on Auto Resize can increase simulation speed, reduce memory use, result in smaller fluid cache files, and decrease fluid render time.