Use the + and - keys to zoom-in or zoom-out. You have 5 levels of zoom, from 1X to 16X. You can see the current zoom-level by toggling the info overlay with the I key. At zoom levels of 4X and above, you will also see the pixel-grid.

New in v1.7 You can also see the color under cursor within the info overlay. To copy the color to clipboard, hit the C key.