The number of rows makes no difference for Union queries. You do need the
same number of fields in each subquery. However, you can do this by querying
only a subset of the fields in the wider table, or by padding the field list
in the narrower table:

SELECT Field1, Field2, Field3
FROM NarrowTable
UNION
SELECT Field1, Field3, Field5
FROM WiderTable

or

SELECT Field1, Field2, Field3, Field4, Field5
FROM WiderTable
UNION
SELECT Field1, " ", Field2, 0, Field3
FROM NarrowerTable

Note in the second example that field types must agree: the example assumes
Field2 in WiderTable is a text field and Field4 is numeric, so I padded with
"" and 0 respectively. You could also pad with Null (regardless of the field
type), but depending on what you're doing with the resultant recordset,
that's not always appropriate.