300 pixel thumbnails usually work pretty well for screen resolutions up to ~ 2000 pixels. We have the Quick Preview and Viewer and can look at large images any time.
If you use 27" or 30" monitors with ~ 3000 pixel resolution you may need bigger thumbnails.
Tip: Under Edit > Preferences > Database you can select a thumbnail size and then click on the magnifier button to see it 1:1 and to get an estimate about the size required to store 1,000 of these thumbnails:
300 pixel: ~ 24 MB for 1000 files.
500 pixel: ~ 58 MB for 1000 files.
800 pixel: ~ 146 MB for 1000 files.
Of course the database space requirements vary, depending on the image content and how effective the compression is. It's a lot of data.
And 200,000 files is a lot, too. I can tell you that large stock agencies don't even attempt to manage this collection size on a single PC. That's rather a job for a server rack plus Enterprise-level disks, IO-systems and UPS. But then, they don't use IMatch
