Hey,
Yes, maxscript isn’t very fast really, try disabling the previewing of boxes and vectors, that should really help.
It actually supports the sizes that you mentioned. The density setting only determines the number of voxels for the longest side, if you scale the bounding box non uniformly you can get whatever size you want, it doesn’t have to be square, it will round it to the nearest power of 2.
Thanks for the feedback! Let me know if you’d like any implemented or bug fixed.