Keeping font size custom when changing window/viewport size

I've got an app that I am trying to use a custom font size in. I am changing the MapMode, WindowExt, ViewportExt, ect depending on limits the user sets.
The font appears fine when the window is first created, but when I go to change the limits, the font does not maintain its size. I was monitoring the
CDC:GetTextMetrics and when the window first appears the values appear to have been scaled( I assume they were scaled correctly since the size is correct ).
I can resize the window with the initial limits and all is fine. But as soon as I change the limits and OnPaint fires again, the GetTextMetrics values don't appear
to be scaled. I tried using CDC:SetMapperFlag but that had no effect. Is there some trick to get windows to keep the aspect ratio of the font?

Thanks for any pointers,

Dan