Dimension doesn't have DPI controls yet so we always export at 72 DPI. You can set the pixel values of the canvas in Design Mode. Select the environment in the scene panel and then check the properties panel for the canvas size property. If you want to increase the DPI, you just need to do a bit of math (we'll add DPI controls soon so you don't have to do that!).
For example if you want a 10 inch print at 300 dpi, then just type in 3000 pixels for your value.
The quality option in Render Mode is in regards to the quality of the 3D result (how much noise or grain is in the image, if we use the best quality of lighting or not, etc.).
You can share ideas, vote, and get status updates on our Feedback portal. Please see this suggestion for DPI controls: http://projectfelix.uservoice.com/forums/552559/suggestions/17849869