You can either set the width of that specific text field:

banddescription:
label: Description
type: text
height: 4
width:75

or

you can do it with css:

input[type="text"], textarea {
    width:100%;
}

It makes the other text boxes also the same width but visually I prefer that anyway:

Great. I'll use that solution. Thank you.

It's fixed.

We need to fixed it in other way, as this method breaks joining fields.

Fixed again.