In Inkscape press Control+Shift+E to export PNG files. Make sure you have "Page" selected or it will autocrop the image to whatever you have selected. (This is actually a cool feature , not a bug)
From the export menu you can change the actual output size. increase this if the edges get too "Jaggie".
Be sure to Save the SVG version as well so you can edit it later. Once it is turned to a raster image there is no reliable way to change it back into a SVG file.
If you ever do need to import a raster image to inkscape, you can select "trace bitmap" from the paths menu. This will make a smooth scaling image, the problem is that it only works well with solid colors.