Ya know what they say, "big feet, big......
Jane has done a GREAT job with that picture showing the Downy and The Hairy MALES side by side for an EASY comparison. Unless you see them together, it can be near IMPOSSIBLE to tell them apart.
The best way way to tell what you have is the length of the bill in comparison to the length of the REST of the head. In the Downy's case, the bill is about 1/3 the length of the head. The Hairy, on the other hand, has a bill that is 3/4 to almost 100% the length of the head.
In body size, the Hairy is almost 50% larger than the Downy. Otherwise the two birds are IDENTICAL.
If you have a very big Downy with a LONG bill, it's NOT a Downy; It's a Hairy!