Is there a way to change the size/width of SugarCube2's textbox?

I'm using SugarCube2's <<textbox>> macro for something like a RPG character sheet, for inputting names and such.
Ideally, I'd like to adjust the width of these boxes so I can control the look and layout of the page.

I've been googling for CSS-based solutions, and added the following to my stylesheet:

input[type="text"] {
width: 50px;
size: 20;
}

textarea {
width: 50px;
size: 20;
}

But nothing affects the size of the textbox! It remains ~300 pixels wide no matter what. Is there any known way to affect the text input styling?