change monster size?

[The chaser]

i cannot change the size in the model editor either.
the buttons are grey and i can't write the number in the text field

Then it's going to be a lot more complicated! You will need a modelling program, Autodesk Maya (the 2011 maya is the most used)
Before doing anything, make a backup from the original monster.

-Go to Maya
-Import the model
-Scale it
-Export it again (as .dae, remember!)

Done. I don't know if you have to re-size the dae_anim's, though.