How to set blob limit while creating column in oracle

You've posted a link to the documentation for Java DB. That's a completely different database engine that happens to be owned by Oracle Corp.

Within the Oracle database, you do not and can not specify a restriction on the length of a blob. You just declare a blob. Either

create table foo(
  blob_column blob
);

or

declare
  l_blob blob;
begin
  null;
end;