what size of a volume can fit in 1 gb?

The built in PolyVox voxel types are 1 byte per voxel, so 1Gb gives you a volume of 1024x1024x1024. Or maybe flatten it to a terrain of 4096x4096x64.

In fact, as I think about it I'm inreasing tempted to introduce this simple volume type, both for threading and performance reasons. Maybe it happens over the next few weeks.