nemam tu pri sebe photoshop ale afaik toto robim zakazdym v cs4:
http://help.adobe.com/en_US/Photoshop/11.0/WSfd1234e1c4b69f30ea53e41001031ab64-76fba.html
pre teba je relevantne:
1. margue tool
2. choose a style in the options bar: Fixed Ratio (Sets a height-to-width ratio. Enter values (decimal values are valid) for the aspect ratio. For example, to draw a marquee twice as wide as it is high, enter 2 for the width and 1 for the height)
3. tahas obdlznik ktory si drzi presny pomer stran
4. copy + paste