The answers given don't specifically apply to a Qualtrics constant sum question. I think the code below is what you are looking for. Change "75px" to the desired width.
Qualtrics.SurveyEngine.addOnload(function()
{
var inputWidth = "75px";
$(this.questionId).select('.SumInput').each(function(name, index) {
name.style.width = inputWidth;
});
$(this.questionId).select('.SumTotal').each(function(name, index) {
name.style.width = inputWidth;
});
$(this.questionId).select('.InputText').each(function(name, index) {
name.style.width = inputWidth;
});
});
I don't believe the specific code above was ever included on the Qualtrics javascript page. I've made a number of Qualtrics javascripts publicly available at https://gist.github.com/marketinview/