At least on my plotter, plotting times come with some jitter. So, let’s say that I mostly see around 150 secs, but from time to time there are over 220 plots as well. RAM drive is usually small, so such jitter may hose the plotter for some time.
Although, I have asked Max to write “chunked” plots that could be moved using his plot-sink. Let’s say a plot would be written in 4 chunks. Immediately, after the first chunk would be written, plot-sink could start transferring it. Once such chunk would be moved, it could be deleted. This way, RAM drive could potentially “double” in size, meaning being a bit more resilient for such jitter.
As far as that other question (how big such ramdrive needs to be), it really depends what drives you have, and where in the drive you are with writing. I recently plotted to Toshiba drives, and 3 drives were good enough to support 150 secs plots, so 4x 72 GB RAM would be needed (3x for plots being moved, plus 4th one that is being built). However, before I plotted to WD Essential (white label), and I had to use somewher 4-6 drives toward the last TB.