Here are the directions for both Photoshop Elements and Photoshop. (Tested in PSE7 and PSCS3)
In Photoshop Elements:
Create a new document and select the Cookie Cutter tool.
In the Options Bar, choose a shape from the Shape Picker drop-down menu.
Click the down-arrow beside the word Shape Options. Select Fixed Size and type in the dimensions you want.
Click once on the document and confirm the shape by clicking the green checkmark.
In Photoshop: to create a shape in the size you want, create a new document and select the Custom Shape Tool. Choose the shape you wish to make.
In the Options Bar, select Shape Layer (the first in a series of three icons on the left.)
Locate the Geometry Options down-arrow next to the shape you've selected in the Options Bar.
From the drop-down menu, toggle Fixed Size and type in your desired dimensions.
Click on the document to create the custom shape.
The Rectangle, Rounded Rectange, Ellipse, and Polygon tools (found in the same Tool location as the Custom Shape Tool) work the same way as the Custom Shape tool.