The way to set the field size is very simple, all you have to do is set the MinimumSize attribute of the field to the desired Width and Height.

MinimumSize=(Width,Height)

For Example: MinimumSize=(200,150)

In case you work with the Process Editor you will need to use the attributes field to add that attribute.

If the size that was chosen does not fit the screen a scroll bar will be shown.

Have Fun