How to resize a plot?

Use AspectRatio -> Full to have the graphic stretched to fill the rectangle specified with the option ImageSize -> {xpoints, ypoints}:

Plot[Sin[x], {x, 0, Pi}, ImageSize -> {600, 100}, AspectRatio -> Full]

To set the size of the image canvas you can use

ImageSize -> {xpoints, ypoints}.

This may not do what you intend, as it will just add white space to fill out the figure to the aspect ratio set by {xpoints, ypoints}. To change the aspect ratio of the plotted regio use the AspectRatio option.