I'm not completely sure whether this is what you are asking, but, if you want to send the paper size to the report as a parameter, you need to use the PAGESIZE parameter, and the dimensions will be in a string of the form '<width> x <height>'.
For example, if you are calling the report from an Oracle Form, you add the parameter this way:
<pre>Add_parameter(YourParameterList,'PAGESIZE',Text_parameter,'15 x 20');</pre>