There are several ways to change the size of an input field.
First is the standard HTML tag. For example, <input type="text" name="MyInput" size=20>. You can then use the appropriate function to send this variable to the host.
The TEDATA tag will send a defined area of a screen starting at the row column values and ending after the defined offset. So, for an input field at screen position row=24 column=16 that is 63 characters in length you would say <TEDATA 24 16 63 AS INPUT>.
From your question, I assume you wish to decrease the size of the input field. So, using the example above, you would simply display less then the full 63 characters of this input field (say 30) like so...
<TEDATA 24 16 30 AS INPUT>.
Displaying more then the number of characters defined in the screen could cause some troubles since the host may not be able to accept a longer string as input.
I hope this helps