Try SPB keyboard for a nice full screen landscape mode. It has a button you push on the keyboard to switch to a full screen landscape mode. The text is entered in a box at the top and once you are finished, all the text that was typed gets entered into wherever the cursor was (ie a text message box - kind of hard to explain, but you'll see what I mean if you try it). It also works well for replying to messages on forums such as these ones - you can see what you're typing without it running off of the screen. There is a
free trial that you can try to see if you like it.
Another way would be to use a program to rotate your screen (for text messaging, emails, etc) and then use your keyboard of choice in landscape mode. I use AEButton Plus and map a long press on volume-up to rotate the screen.