You can resize a datafile, But I dont think that is the real problem with your database.
Your SYSTEM tablespace is getting way too big. It looks like some objects are getting created in there that should not. I cannot see a reason why SYSTEM would grow to 23GB, since it is only used by oracle to store internal data like object definition and sometimes some repository.
I would look at the objects in SYSTEM and move what does not belong to other tablespaces like USER, TOOLS, DATA or INDEX depending on the object.
Also I would check the default tablespace for the users, looks like it may be SYSTEM and should not be.
In any case, you can also add smaller datafiles to SYSTEM to solve your problem instead of increasing the size of the dbf you already have.