in edit or create mode, when a field is merged with the right one, problem of size

the layout is a two columns layout.
If I put the "dc:title" field on one column, putting 100 for size value behaves nicely.
If I put the "dc:title" field on two columns thanks to "merge cells", putting 100 for size value is not nice because in that case the label ("title") become as large as the input field.