Hairy Woodpeckers have a larger overall size and shape reminiscent of a Red-bellied Woodpecker. This bird looks nothing like that. Hairy Woodpeckers also typically have a shoulder spur, while this bird has a black looking mark, it's too low to be the shoulder, and looks more like displaced feathers. Lack of tail spots *can* be a feature of Hairy, but like apparent bill length, is more of a supporting aspect, rather than defining, as some populations of Downy Woodpeckers also lack tail spots.
To show how feathers or lack thereof can affect how long a Downy's bill looks, the following three birds are all CONFIRMED Downy Woodpeckers. Look how different the bills look.
downy on suet by midgetinvasion, on Flickr
Woodpecker on tree by midgetinvasion, on Flickr
woodpecker by midgetinvasion, on Flickr