Mailchimp text field height

I’m trying to set the specific height for one filed (MMERGE7) in my signup form, I’ve added the following to custom css:
#mc_mv_MMERGE7 {
height: 100px;
padding: 0 0 80px 5px;

However, it didn’t work.
What I want at the end is to add not a text line but a paragraph box, so the subscribers would be able to leave a comment (probably, a long one).

Hopefully, there is a way to do that:)