Thumbnails can be great for working out large simple images. Or even large complicated images like the NAIL TPB cover.
... But most often I prefer to work them out at print size. Tracing through the logo, if it is available. These are sketches for a Titan Books reprint of Alien Legion and an issue of Aquaman.This was a rejected Idea for the third issue of Another Nail. There was also some concern about the title and my alternate suggestions are above.The other was intended for Marvel UK's Doctor Who book.
I will sometimes go back to working story pages out as thumbnails (if I'm drawing a new book, new characters-- or just for a change of pace) but, as writers tend to think crowds and armies are essential to every story, working at a small size becomes counterproductive so I usually work on full sized roughs or, if I'm feeling confident, directly onto the art board.