Sometimes it is easy to tell the difference between a smaller Downy and larger Hairy Woodpecker but here are some woodpecker pictures showing how to tell the difference easily between the two woodpeckers without going just by size. A large Downy Woodpecker can sometimes be mistaken for a young, smaller Hairy Woodpecker.
The easiest markers are bill length compared to skull length. The bill on the Downy Woodpecker is much shorter than the skull length while the Hairy Woodpecker bill equal in length to the Downy Woodpecker. Also, the underside marker feathers on the Downy Woodpecker have black stripes while the Hairy Woodpecker has no black stripes.