Iīm new on Mac and I canīt find a way to see a large preview in the Finder. I want to see large previews so that iīm able to see what images are keepers, so that I can delete the others before I import to LR.
In 10.4's Icon View mode you can zoom up to 128 pixels in the view options for the window. You can also control click on a group of files and have it display a slideshow/contact sheet sort of interface. This doesn't let you delete things directly, but you can add them to an iPhoto library as a temporary holding tank prior to DNG conversion.
10.5 should improve upon all of these things, but for preliminary weeding I think the present implementation is adequate if you don't want to do it with Lightroom for some reason (and I'm not 100% sure what that reason is exactly as you don't have to copy any files anywhere to add them to Lightroom and you can yank the entire collection out pretty quickly once you're done with them anyway).