Grid fields are very user-friendly, as you can show multiple columns. Settings the width is done via CSS casses, that you need to add to 'customer.css'. You can set the width per column (in pixels). I will post some examples below.
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.Grid3Columns table colgroup col:nth-child(2) { width: 100px; }
.Grid3Columns table colgroup col:nth-child(3) { width: 200px; }