Hi Bruno,
Try:
.orbeon .fr-grid .fr-style-surname input { width: 100px; }
Two things:
1. The class you specify on the xf:input goes on the span element which is around the input.
2. There is already some CSS specifying the width, so you need to make sure that your rule has a higher priority, hence the additional class names in the above selector.
To figure this out, your best bet is to use the Chrome Dev Tools, or similar.
Alex
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