There are two options I know of.
1/ In your numeric question, click Advanced | Numeric Settings and enter the width there. Hit OK to save.
2/ Insert the following CSS script in to your footer ...
<style>
#[% QuestionName() %]_div .numeric_input {
width: 200px;
}
</style>
Change the 200px to your desired width.
Note: the above CSS script is something I have used in the past courtesy of Zachary Anderson (he provides wonderful support on this forum on a daily basis).