Some observations and results from playing with the source from jonwil
and freemangordon:
(HildonVKBRenderer, HildonIMWesternFKB, Keyboard Layout XML)

a. HildonIMWesternFKB responsible for the GtkDialog, and some more
b. HildonVKBRenderer renders Keyboard layout inside
of the GtkDialog

1. Bottom button row with menu, space and enterkey is created by
HildonIMWesternFKB with fixed size. This is the reason why the
space button overlaps the menu button if you just use the landscape
keyboard with forced rotation.

2. Size of the other buttons is defined in the layout xml files ...

3. ...whereas the shift and backspace button use a theme based image
The name of the images is defined in the himrc resource file
(/usr/share/hildon-input-method/himrc).
Thats the reason why the arrow in the shift button vanishes
(scaled to zero) if you use the landscape keyboard in a dialog with forced rotation.

4. Font is defined in the himrc resource file: X-LargeSystemFont

5. The drawing area for HildonVKBRenderer is fixed at construction
time of the HildonIMWesternFKB dialog 800x210 pixels.
Thats the reason why the dialog looks ugly if you use a user defined
xml layout file with more than 3 rows (cut-off/repeating button pixmaps)

6. Predefined LayoutTypes in (western) keyboardlayout xmlfile:
"NORMAL": Not used in Fremantle, the old stylus/finger keyboard
with full qwerty and number block
"THUMB": Normal keyboard layout with 3 rows.
"SPECIAL": ?


Todo

I. Let WindowManager know HildonIMWesternFKB supports Portrait mode
Done/Works

II. Listen on orientation/size change
Done/Works

III. Resize space key button on portrait orientation
Done/Works

IV. Make drawing area dimension of HildonVKBRenderer changeable and
recreate preallocated pixmap if orientation changed
Done/Works but depends on how we want a portrait mode layout
look. What dimension should the portrait mode keyboard have?

V. Load a "portrait mode layout" for portrait orientation.
Done/ I am using the unused layout type "SPECIAL" (as mentioned above) and define a layout for this type in my de_DE keyboard layout
Is this the way we want to use a portrait mode keyboard or
should we use the same landscape layout but using a different way
rendering it (dont know if t his is possible)

Some experiments
VI. Change font to X-SystemFont for portrait mode
Works/ But that way we would need to add more resource descriptions
to the himrc resource file

VII. Change button icon name for shift and backspace icon for
an icon with smaller dimension in portrait mode
Works/ But that way we would need to add more resource descriptions
to the himrc resource file and add icons for all themes.

tl;dr
Need some help, please create some mockups for portrait mode
keyboard, but notice the menu/space/enter buttons on the bottom
are fixed and not defined by the keyboard layout xml. The
shift and backspace use a fixed size icon of 72x70 pixels.