> If I change a varchar size from 15 to 11, for example, and
> there´re some rows containing up to 15 positions,
> what happens with this ones?

It is not possible using standard DDL commands
to shrink field size (only create new column and copy data over)

However, you can do it by updating system tables.

> Do I have to cut the extre chars?

Yes, first cut extra chars, then mess with system tables.
(Otherwise cells with too long data become unreadable.)

Ivan