Can you post the EXACT steps that you are using?
Are you trying the export using SQL Developer? Can you provide the create statement you are using for testing.
I created a DOMAIN called 'OneHundred' which is defined as a varchar 100, Characters.
In my relational (Oracle 11g) model, I create a new table, with one column, of type 'Domain - OneHundred.'
When I preview the DDL it's correctly defined as ... VARCHAR2 (100 CHAR)
What are you doing differently?
Also, don't forget to tell us which version of the tool you're using.