Hi Safiya,
You should be able to set the word length by changing the data types for the specific ports/values inside the discrete transfer function. You can set it in the "Data Types" tab in the block parameters. You can specify for each the word length using "fixdt".
For more information, see the "Parameters and Dialog Box" section of the following link:
As far as output resolution is concerned, this is really application dependent. You can try the Fixed-Point Advisor:
Hope this helps you get started,
-Adam
