You will need Total Commander for this tip to work.

1. Launch Total Commander

2. Browse to "Windows" folder

3. Find the file "thread_sms.htm"

4. Press the icon at the bottom row, next to the Show button.

You will see a whole chunk of code

<Inbox:content id="COMPOSESUBJECT" width="98%" height=35 spell ......

Locate the line shown above and modify the value of height to something you like.

I have change the default value of 35 to 65, this gives me a nice 3 line spacing for the input :)