In Photoshop... under the File menu you want to select 'New' which opens a box that allows you to create a new document. In this 'new' box the top three options allow you to asign width, height, and resolution in various measurements. You want pixels for width and height, and 72 dpi for resolution.
Create & name your new document. Open the image you wish to copy and paste and size into the your new PS document.
I like to use the rectangular marquee tool to select the image, but you may use whatever copy & paste method you're comfortable with. Once you paste your image into your 'new' PS document, you may have to resize it to fit your new PS document guides.
Save your document as is before attempting the scale change.
Under the
Edit Menu, select
Transform then select
Scale.
The image as it was pasted into the new document will originate in the top left corner of the image area.
Read this before trying it: You will need to click on the square 'corner' of your image - holding down the mouse button as you do this, and the shift key! Now 'drag' the image down (or up) in size to fit the preset guides of the PS document. Once you have it where you want it, click on the rectangular marquee tool in the tools window and a window will open asking to apply the change... you say apply, perhaps move the whole image into the guides... and then hopefully save the image in the format you wish.
It may require more manipulation than that...
but those *should be* the very bare-bone basics.
*unless, of course I need more coffee this morning*
-kells-