Very, very, large
Over the weekend, Microsoft Studios' Mike Ybarra tweeted out a pair of photos demonstrating how achievements will be represented on the Xbox One. Instead of a simple list, there is a less helpful grouping of huge images, allowing a rather limited number of achievements to be displayed on screen at one time. The second image shows a full-screen view of a single achievement, presented as a massive wallpaper-style image with the award conditions overlayed.
Pretty? I suppose. Isn't this really impractical for the people who seem to most appreciate the act of earning achievements? I would think it much more useful to have a list option if I were going to attempt to maximize my Gamerscore.