I edit on a 1920x1200 NEC monitor and I have the 1:1 preview settings set to 2048, high quality.
I don't have a definitive answer but am curious about responses as my catalog is similar to yours but running on much lesser hardware.
One point of clarification though... the above settings are for STANDARD previews, not 1:1. 1:1 previews are created based on the actual pixels in the image file. The preview is the exact same pixel count size as the actual image. Making 1:1 previews speeds moving between images in the Library but it doesn't help in the Develop module (which is where I feel I really need something like that). You may be needlessly creating 1:1 previews and that in turn may be seriously bogging down overall catalog performance. Creating standard previews and moving the quality setting down a notch or two would probably dramatically improve your catalog performance while causing little, if any, issue in terms of evaluating images. Your preview file would still be large but much more manageable in size. My catalog contains about 170k images. Most of my previews are 1440 medium quality versions. My previews file is about 60GB.