OK, I'll try to explain.
Make your text layer.
Make a clone of the text layer (not a duplicate) and put it below the main text layer.
Apply a Simple Border filter to the cloned text layer. Set the colour to something you can see - the default is black so if your background is black you want know what's going on!
Set the Simple Border width parameter to 100.
You will now have a box that exactly matches the main text layer whatever you do to that layer, whether it's inputting different text or scaling it or whatever.
The only problem is that it exactly fits the text object with no extra space but that's easily fixed. All you need to do is go into the properties for the clone layer and set the scale and position parameters to whatever you want.
If you want to be perfectionist, you need to offset the anchor point Y position so that any scaling you do is proportional, otherwise you will need to keep adjusting the position as you adjust the scale. If you set the anchor point right you can adjust the scale only and the position will take care of itself.
That's it.
I forgot to add that you'll need to set your text justification to centred for obvious reasons.
Simon Ubsdell
http://www.tokyo-uk.com