Luckily there's an easy fix. Using your Reg. Editor (I use Total Commander), navigate to the Windows directory on your mobile device and find the file called thread_sms.htm. You will need to edit this file (clicking on it will only cause your device to try and open it in your browser; make sure to select the edit icon in your Reg. Editor to alter the file).
Scroll down and find the line that looks like this:
< Inbox:content id="COMPOSESUBJECT" width="98%" height=35 spell focus count links noclearonbackpresshold limit=0 autocap inserttab scroll / >
Change that "height=35" to something larger. I've got mine at "60" and that seems to work pretty well.