I've made many (dozens) of 24x36" from D3 and D2x files. That's as big as my printer can make without going into panoramas. I've made TWO 30x45's - actually one of them is more like a 30x40, so it's effectively even bigger. They're both for customers, and they liked them to buy them. I think they came out just fine. A 24x36 is a huge print when it comes off the printer, but once it's framed and in a large space, it can look surprisingly small.
Pretty much all of my large files were captured from top-caliber glass, mostly macro lenses, 24-70, 35-70, 70-200, 400/f2.8 etc. The one exception I can remember is the Sigma 12-24, which is good but not at that same level. Most but not all of them were shot either at high shutter speeds or on tripod/Mup/remote.
I don't think I'd care to be printing any of my own files much bigger than 30x45 or so, but I'm not that good at printing and I probably have a lot to learn about post processing too. If it were me, anything 60x100 must have been either a stitch of many files, or it was from a very big sensor.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!