New update has been posted
here to address this issue.
Scale Mode
A new scale mode was added to the Octane Volume (OpenVDB) Effect.
Fixed Scale - This mode preserves the original scale of the
VDB file. The proxy still provides position, rotation, and scale but it does not force the volume to be rendered within the borders of the proxy. A slider and two edit boxes on the new Scale Settings dialog allow you to scale the
VDB volume to fit within your scene. Since each
VDB file can provide its own initial size, a loaded
VDB volume can appear way too large or too small for immediate use. The min-scale/max-scale edit boxes allow you to provide a 'reasonable' range for the Scale slider. Once a min and max scale has been entered, the scale slider can be used to pick a value between the min and max values. This should allow you to more easily tweak the size of the volume to better fit your scene.