The text field configuration should allow you to set the size of the input element:
It appears that there is a minor bug there and the width unit is always px even if you select something else. But it does change the width of the input element.
If you have an earlier version, this part of the configuration may be missing. In this case you can use css. You need to find the selector path of the element (you can use the browser inspect) and add css in the css tab of the activity to the desired effect. You can also enclose the field pattern in the template with an identified container element and use that container element in the selector path.
So, for example, assuming the selector path to the text input element is
.dataformview-grid.Grid .entry .generaltable tr:nth-child(4) input
the css code in the css tab of the activity could look something like: