Hi Kala,
If you want to load ASCII character string to certain column, then you can define varchar column type as well, the load data is UNICODE character string to certain column, then you can define nvarchar column type as well.
Besides, if you want to use text search feature, then you can define shorttext column type.
anyway, the column type depends on your business definition and purpose.
Regarding as column type length, you can define the column data length based on your business data.
or user's requirement, general its length depends on history business data or user's requirement.
You can refer to below link.
http://help.sap.com/hana/SAP_HANA_SQL_Reference_en.pdf
Hope this can help you.
Regards.
Jerry