I think the answer is “it depends” - on various things, like the volume of data you’ll be ingesting into Graphite, etc. Maybe others on here have experience with such a setup and can advise.
My personal advise would be to just not overthink it and pick something (16GB? 32GB?) and see how it goes. Have in mind that with VMs these days, generally speaking:
(a) your HDD image will be stored in a sparse format, and will only take up as much space on the host as there is data stored on it, rather than the full allocated amount
(b) you can usually increase the allocation at a later point if it turns out to be too little.
So, unless you have specific constraints, I’d say that the benefit of spending a lot of time up-front to optimize the size is probably fairly limited.