I think the export presets in Aperture can do what you want.
You just would have to define a custom export preset.
Assuming the images you want to convert have been imported to Aperture, select them in the Browser and select to export the versions (File > Export > Versions).
In the Export panel select a png export preset, then switch to "Edit"
In the "Edit" panel duplicate the png preset, then set the pixel size you want to use and export your images.
If you need to do this with images that aren't in Aperture, this is the sort of thing that Automator was built for. Look at the "Scale Images" and "Change Type of Images" actions. There's a step-by-step on how to change file types with Automator here, you should be able to pop in a "Scale Images" action into the workflow to add that function.
Message was edited by: Aye Es Oh