This once also looks very simple, that you drew a Rounded Rectangle in Photoshop using Rounded Rectangle tool. Then you press Ctrl+T (Transform) and then change the size. Remember by doing so Corners Quality may disturbed. Here is the simple technique how to change the size of a Rounded Rectangle in Photoshop.
1. Take any new file.
2. Pick Rounded Rectangle Tool 
(You will find this tool inside Rectangle tool 
)
3. After selecting Rounded Rectangle Tool (before drawing any object), don’t forget to click the icon Shape Layer 
(in the top property bar), otherwise Paths icon 
would be drawing only Path with no physical objects.
4. Now draw any rounded rectangle, suppose you have drawn a rectangle which needs to be resize. You don’t want to make other rectangle, suppose you have applied styles on it. So here technique starts.
5. Pick Rectangular Marquee Selection tool 
.
6. If you want to change the Width of Rounded Rectangle then drag the mouse from outside of the Rounded Rectangle and select half of the rectangle width wise like:
7. Now select Move Tool 
. Now you can use Arrow Keys or Mouse to change the width of Rounded Rectangle.
8. After changing the size of the rectangle now again pick Rectangular Marquee Selection tool 
. Select some part of the Rectangle (vertically) in the middle of Rounded Rectangle. (Far away from Rounded Corners please).
9. Now again select Move Tool 
, press Right Arrow once then left arrow once (it will separate that part of rectangle):
10. Now press Ctrl+T (Transform) and then change the width unless it touches the right part, (or even it may go inside right part) and then hit <ENTER> key. Then Ctrl+D to unselect.
11. Some time spent but nothing lost, same corners Rounded Rectangle remains. Like this you can also change the Height of rectangle. Just remember one thing that don’t select the Rounded corners part, always select far away from Corners. Enjoy!!!
