Hi there Renier,
I am not sure how you mean.
The form is inside mailpoet.
Since CSS generally depends on the specific page in question, we were requesting a link to the page on your site so we could view it and try to provide a CSS snippet for you.
Re-checking your site, I see the form is actually in a column of your site’s footer, however it looks like you’ve removed the field you were trying to change.
First, for that field, you will want to add it as a “Text Area”, rather than a “Text Input” — text inputs are usually just one line, while text areas can be multi-line and grow as necessary:
Link to image: https://d.pr/i/cWvS1B
After that, in the block settings you will be able to choose the number of lines that you want it to show:
Link to image: https://d.pr/i/sbd5cI
If you were referring to the width of the field, from your screenshot it looks like it is just fitting within the available width of that footer column. If you are still having trouble, please add the field to the form and let us know so we can view the page itself with the field added.