Coffee has to be dominated by pictures, IMO. That's how I would make the decision on trim size. What is the majority of the content- words, pictures, or both? Is it going to be full, high quality color? Coffee books are also closer to square. Most commonly around 10in x 12in, or thereabouts.
Workshop manuals or usually closer to your A4 size reference. Considering the subject matter, that could be an interesting artistic choice. You could even do up the cover graphics like a workshop manual. It could combine color and B&W images throughout the book. If I went with this format, I would consider what's called a trade paperback. It's a thicker, full color cover usually in gloss or satin sheen or the combo of both. I think you could do trade paperback in coffee or the 8in x 11in "workshop manual".
Considering this isn't likely to be a mass production deal, I wouldn't even consider going small paperback. It'll be cheaper, sure, but I think the story and images wouldn't get the proper setting in a small paperback.