A bit nervously, I overnight scanned my largest object to date --- a crisp (okay, not really) 50 dollar bill. The lab is locked but there's always the chance that it'll be opened in the morning for a tour or something. So I was a bit relieved to find that it was still there.

Unless you're doing active focus correction or Z positioning, the trick is to just make your object as flat as possible, and use the minimum acceptable magnification. I use two small strips of double sided carpet tape to keep it stable during the scan. 

The high res image was 17,000 images, and I haven't even attempted that yet, but...

The low res scan worked pretty well! 738 images, approximately 50% overlap, originally 480/640 pixels each, stitched in microsoft ICE. The mechanical precision is sufficient, I think, that you could just overlay these images on top of each other and get a decent image without using any special software. 

(The above image is 25% scale after you click on it.)

All sorts fun microtext scattered throughout.

Full image: https://www.easyzoom.com/imageaccess/87fadcf548ca4d4892ffe8383ad1fe66