I am creating a z-depth map in Poser 8 using the sillhouette and depth method.
My problem is that after floating the preview window,when I try to resize it to the same dimensions as the image I want to use it for, it will not go to the size or aspect ratio that I want.

Is there a way to render an image the way that it is displayed in the preview window?
I know that there is Avanced Render Settings 2 over at RDNA that can create z-depth maps, but would like to do it without having to fork out for a plugin.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches

You did try Match Preview Window under Render Dimensions?

Render Dimensions -> Specific size -> uncheck "keep proportions" (or whatever it is) -> enter dimensions.  If you didn't move the camera then the image should come out the same resolution and aspect ratio.

Match Preview isn't going to help him, since he undocked the window and the window probably doesn't have the same aspect ratio any more.

My Freebies

I think that I need to clarify what I am asking.
I am using a technique from years ago to create a z-depth map in poser.

Make sure all figures and objects are white (not sure if this is needed)
Set the background colour to black.
Set document style to sillhouette.
Select depth in display.
Delete all lights.

The preview now shows a z-depth map that can be saved and used for DOF as well as other things. Unfortunatley I don't know of a way to render this type of image and I cannot  make the preview image (thus the image saved from the preview) to the dimensions of the main image that I will be using it on.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches

Example of the kind of thing I want to achieve but at a larger size than my preview window wants to go (even when undocked).

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches

I think I understand your dilemma.  Have you tried rendering your preview window?  Click the Render Tab and change the Firefly setting to Preview.  Afterwards, render as normal.  That should produce your Z-Depth map to the same size as your actual render.

hborre thankyou so much!
I had been looking in the render settings from the top menu bar and couldn't find what I was looking for. I had no idea of the different options on the render tab.
I still find it harder absorbing e-instruction manuals etc than hard copy ones, otherwise I might have known that.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches

I actually caught that trick from a previous post where a user was rendering preview and couldn't understand why the renders were....not rendering. LOL!