Jose,
What data type gets created in your database will depend on the encoding you are using. If your database is not using a Unicode character set, you can create a varchar field by choosing Text and specifying a length of up to 4,000 from ArcGIS Desktop. If you go over 4,000, a CLOB field will be created in the database.
If your database is using a Unicode character set, it will depend on what specific language you are storing. Characters in Latin-based alphabets use 2 bytes, but there are other character sets that use 3 bytes and some use 4 bytes. As long as you stay under 4,000 bytes, though, an nvarchar field will be created in the database. Above that, an NCLOB is created.
As Vince said, though, I would think you'd want to set a comment field at a length lower than 4,000 bytes, but I suppose it depends on your particular application. 🙂