How do I set the size of a field at runtime?

I've tryed the following...

----

dim db as database
set db=OpenDatabase("somedb.mdb")
db.TableDefs("sometable").Fields("somefield").size = 90




(this returns Error 3219, Invalid Operation)


db.Execute "ALTER TABLE sometable ALTER COLUMN somecolumn text(90)"




or


db.Execute "ALTER TABLE sometable ALTER COLUMN somecolumn c(90)"




(this returns syntax error in ALTER TABLE statement)

how can I do this from VB?? it can be easily done from Access...

Thanks in advance