It rather depends on what your project consists of and how much detail you wish to see in the preview. You might start with 1024 or 2048 and see how that works. If you are displaying at Preview - Auto that may be enough, depending on how big your selected area is you're looking at. When you set the Preview quality higher, that will use more memory and will store less of a selection in memory. You could use up to 7044 MB, but you will probably want to set that back down to something small like 250 or less before you render a project. Play around with the settings and see what works best. I hope that helps!