When converting xls to DBF table, can I change the length of text fields?
No. Excel does not enforce field lengths or types, so ArcGIS makes some educated guesses. For type it uses the values it finds in the first few fields, and for length it is based on the assumed type. For text fields it sets the length to the max possible length of the field which is 254 characters, to limit truncated values by assuming a shorter field-length.
To force a specific length, you could create a temporary table with your defined field lengths, append your Excel data to that temp table, and then write that temp table out to your DBF file.
From Understanding how to use Microsoft Excel files in ArcGIS
Excel does not enforce field types for values during data entry like
standard databases do. Therefore, the field type specified in Excel is
not used in determining the field type exposed in ArcGIS. Instead,
field type in ArcGIS is determined by a scan of the values in the
first eight rows for that field. If the scan finds mixed data types in
a single field, that field will be returned as a string field, and the
values will be converted to strings.
And from Formatting a table in Microsoft Excel for use in ArcGIS
ArcMap can only read the first 255 characters of a cell. If you have
more characters than that, ArcMap converts the field to a BLOB type
and you won't be able to read its contents.
