You can create your own extension of the VarcharType class and override
the toDatabaseType() method to change the behavior of what is generated.
Without adding a new class, the easiest approach may be to use
changeset parameters to define a "charUnit" variable.
In your changelog you can define:
<property name="charUnit" value="" dbms="pgsql"/>
<property name="charUnit" value="CHAR" dbms="oracle"/>
then you can just use <column type="varchar(255
${charUnit})"/> and liquibase will substitue in CHAR or not
depending on the target database.
Nathan