There isn't an easy way to make a PDF of a given size. The style dialog has a control for the size, but the PDF produced has been resized to fit the paper, so it assumes the scientist will resize again when the image is inserted into a poster.
There may be a ticket for this, but I wish I had fixed this a few months ago, and I don't want to forget about this after AGU.
Also it occured to me that there's a nice way to get precisely-sized plots, by turning off autolayout, and setting the marginColumn to 4em,4em+360pt to get 5in wide plots.
Last, sometime earlier this Fall I moved the control points for resizing and moving the plot edges, but this was not completed and the click position is the old position.
I think I'll support 4em+5inch, which would be translated immediately into points.