The image will always have the same dimensions as the screen or window being captured — a 1440x900 screen will create a 1440x900 screenshot.
Today's operating systems are resolution-dependent: everything you see screen is either being shown at actual size (e.g. a 10-pixel-by-10-pixel icon which has actually drawn as 10 pixels by 10 pixels) or has been rendered at some pixel dimensions. So, the size of objects on a computer display is entirely dependent on the pixel density of the display — a higher-resolution screen (or screen capture!) would make everything smaller.
An interesting example of this is Apple's iPhone 4 — its screen has a higher resolution (and pixel density, since the screen is the same physical size) than previous models. Apple had to craft bigger icons to keep everything the same apparent size, and screenshots from iPhone 4 are much larger than older models.
You can change the format of screenshots, and the folder where they're dropped, using utilities like TinkerTool.