Yeah, new user limitations, sorry.
I replied on the mailing list, but for future users who find this and not that topic:
I’m not familiar with vector graphics export, but for PNG images (maybe tiffs, I haven’t tested it) the OSX application Preview can help you fix the DPI. Just export to the number of pixels you want and then fix the DPI in Preview. (The control to adjust DPI can be found in the dialog created by Tools → Adjust Size)