Recent publication announced that a whole zfish brain has been imaged at electron microscopy resolution (EyeWire Mystics map a region of zebrafish brain) and this is the first entire vertebrate brain to be imaged at EM Automated synapse-level reconstruction of neural circuits in the larval zebrafish brain | Nature Methods
This review article has a summary of the largest volumetric datasets out there. At the top is MICrONS, the dataset that will become Pyr Mapping of the zebrafish brain takes shape | Nature Methods Even though it is just a piece of a mouse brain (cubic mm) you can see it’s over 10x larger than FlyWire