Diskinfo does not have an option for specifying the size. You can do that with diskpart though. Look at your osd script and you will see other calls to diskpart scripts. Setup a similar script and execute diskpart referencing a new script what will run the extend command. The diskpart script would look something like this:
select volume V
V will be the volume number you want to extend.
N will be size in megabytes to add to the current partition.
Then place the script somewhere the osd client can access it. Could be put in the wim, or just put in the folder where the image file is. Then I can call diskpart instead of diskinfo.
REMEXEC40=diskpart /s i:\extendpart.txt
In this example, i: is where the image file created a mapping to and the diskpart script file would need to exist there also.