You can't in Photoshop -- most other PNG implementations fail on larger images, so we haven't increased Photoshop's PNG format limit beyond 30k.
Photoshop has some old 15 bit limitations in some of the implementations of its formats, and I know it prompts you with messages of "Images greater than 30,000 pixels on a side may be incompatible...".
Perhaps you've hit one of these limitations (I'm not sure about the PNG implementation, I haven't experimented with it specificially myself).
I do know that Adobe is aware of these limitations and that we've reached a time where computers are now big enough to easily handle gigapixel+ sized images.
Edit: Disregard - I see Chris has answered definitively in the time I took between reading your post and posting a response.
@Chris Cox this sucks so bad
you know why ? because there are third party softwares sites that converts your TIF to JPEG or PNG and you don't support it
I was amused that how photoshop easily handles such big image but now totally disspointed that you cant export it with favorite formats
Sorry, but we get more complaints when we export files that most other apps can't read.
And CS 6 does support JPEG files up to the limit of the format.