Found the manual? Found the help? What have you done so far?
From what I know about Aperture (seems similar to Lightroom) the size of the image is defined upon exporting (after finishing all editing). There you can define the size and resolution.
U can either do up a export preset, or resize thru light-table, or if ur intention is to email it, thru Aperture/ Preferences/Export/Choose Email images.