Per excellent tech support... I relay the info here for all users:
This field is not used by default x-cart, thus you need to modify
/xcart/include/product_modify.php and replace
#
# Update product data
#
Code:
db_query("UPDATE $sql_tbl[products] SET product='$product', descr='$descr', fulldescr='$fulldescr', avail='$avail', list_price='$list_price', weight='$weight', productcode='$productcode', forsale='$forsale', distribution='$distribution', free_shipping='$free_shipping', shipping_freight='$shipping_freight', discount_avail='$discount_avail', min_amount='$min_amount', return_time = '$return_time', low_avail_limit='$low_avail_limit', free_tax='$free_tax' WHERE productid='$productid'");
with
#
# Update product data
#
Code:
db_query("UPDATE $sql_tbl[products] SET product='$product', descr='$descr', fulldescr='$fulldescr', avail='$avail', list_price='$list_price', weight='$weight', productcode='$productcode', forsale='$forsale', distribution='$distribution', free_shipping='$free_shipping', shipping_freight='$shipping_freight', discount_avail='$discount_avail', min_amount='$min_amount', return_time = '$return_time', low_avail_limit='$low_avail_limit', free_tax='$free_tax', dim_x='$dim_x' WHERE productid='$productid'");
As you can see I've just added , dim_x='$dim_x' to the list of saved attributes.