Are you 100% sure you have added the code to the correct form?
If so, you can try adding the !important flag like this:
td.ms-formlabel{
width:200px!important; // Set to the preferred width
white-space:normal!important;
}
You can use the developer tools to see what style is applied – right click the item you want to look at and select Inspect – now you can see the style in the dev tools.
Alexander