problem with table cell-width

I have a problem trying to format my table cells using numeric values.
As a formatter I use fop-0.93.

When I use numeric values for table cells all cells will be the same
width. But I want to use a different cell-width for each cell. The
layout of the table is fixed.

<tr><tag>Do</tag><datum>12.06.07</datum><uhrzeit>15:00 - 16:30
Uhr</uhrzeit><kursnr>Kurs-Nr.:
</kursnr><nr>10</nr><beitrag></beitrag></tr>

<xsl:template match="tag">
 <xsl:param name="pos_tr" />
 <xsl:variable name="pos_tag" select="position()" />
 <fo:table-cell column-number="{$pos_tag}"
xsl:use-attribute-sets="table.contents">
 <xsl:attribute name="width">7mm</xsl:attribute>
 <fo:block>
 <xsl:attribute name="text-align">left</xsl:attribute>
 <fo:inline font-weight="bold"><xsl:value-of select="."/></fo:inline>
 </fo:block>
 </fo:table-cell>
</xsl:template>

I can enter several values for width and none of them work. Inside the
FO-File the values are at the right position but the formatter does
not include them.
Any idea so far?