I asked this question back in june
https://asprunner.com/forums/topic/30575-Problem-setting-heigh-of-text-area-in-dialog

The solution turned out to be very simple.

By adding some css to the event that calls the popup (in my case a button).
If it's being called by a control value change event, then add the css to the form - use inspector on the textarea in question to find the values.

//to adjust height of textarea in a dialog.
//Use the name of the function, the control id and set the height using the css editor for the form.
// control_x is the control[0] number from top to bottom.
`

btnName_control_1 {

}
`