Changing a third party resource (such as a .css file in EPiServer) is never a good idea and you describe the main reason.

It sounds like you managed to change the size, except that your change was made in the sleek.css file?

You can add this to a custom .css file and register it as a module.

Read more about modules on http://world.episerver.com/Documentation/Items/Developers-Guide/EPiServer-Framework/7/Configuration/Configuring-moduleconfig/

A quite simple example of this is the module.config in a sample Alloy Templates

Hey, are you talking about admin or edit mode? 

If you are talking about admin mode, which Name Textbox do you think of?

If you are talking about edit mode and the content Name, you might consider using an editor descriptor:

using System;
using System.Collections.Generic;
using EPiServer.Shell.ObjectEditing;
using EPiServer.Shell.ObjectEditing.EditorDescriptors;

namespace PROJECT.Web.Business.Edit.EditorDescriptors
{
    [EditorDescriptorRegistration(TargetType = typeof(string))]
    public class StringEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            base.ModifyMetadata(metadata, attributes);

            if (metadata.PropertyName == "icontent_name")
            {
                metadata.EditorConfiguration.Add("style", "width: 1500px");
            }
        }
    }
}

This is the code for EPi 7.5, so might be that some namespaces are different for version 7.

Thanks for the fast respond.

I will reply if i have further question around this topic, in the mean time, thanks.