(Please read intro before asking any questions, thank you.)

Okay, so everyone knows how to do it, big deal. But some also didn't know how to. So, I'll post on how you can open a new file in GIMP 2.6.

I won't write much, don't worry.
First off, choose 'File' and choose 'New...', a box (image 2) will appear and you can adjust the size under the 'image size'. And you can also choose the measurement type in the box with 'pixel' word in it.

Now, you can see the two small picture beside the '640 x 400 pixels', the right image is 'Portrait' and the left one is 'Landscape'. You know what are they for. :D

So, you saw the 'Advanced Options'?
That's where you can choose whether you want an image with foreground/background color, white or even transparent fill.