I've found out how to change the amount of space available to write text messages. The answer was on geekedout.se, so I can't take any credit for it myself:
Launch Total Commander. If you don't have this file manager installed, you can get it here:
Total Commander - PocketPC. It's free.
Browse to the Windows folder
Find and select the file: thread_sms.htm
Tap on the little Notebook icon in the bottom toolbar of Total Commander to open it
Locate this line of code: < Inbox:content id="COMPOSESUBJECT" width="98%" height=35 spell focus count links noclearonbackpresshold limit=0 autocap inserttab scroll />
Replace the original height parameter "35" with "85", for example.
Press save and do a softreset (restart your phone). Voilą!
Alternatively, you could connect your phone to a PC with ActiveSync, copy the file to your computer and edit it there with Notepad. Just make sure you have Explorer showing hidden/system files. You might have to overwrite the original file with Total Commander (or Resco Explorer, my favorite) since it is part of the ROM and therefore protected by the system. You can also download my already modified version of thread_sms.htm here.
Check out
GeekedOut.se : Guide to Windows Mobile it's got some great stuff on it!