Retorna tamanho de consumo do campo da tabela

SQL: 

execute block returns (tamanho numeric(18), TABELA varchar(100), CAMPO varchar(100)) as
  declare variable xsql varchar(8000);
begin
  for select trim(rdb$field_name), trim(rdb$relation_name) from rdb$relation_fields into :campo, :tabela do
    begin
      :xsql = 'select sum(bit_length('||:campo||')) from '||:tabela;

      execute statement :xsql into :tamanho;

      if (:tamanho > 0 ) then
      begin
        suspend;
      end
    end
end

Retorno: 

| TAMANHO | TABELA                     | CAMPO                       |
----------------------------------------------------------------------
|     192 | NOMETABELA                 | COLUNADATABELA              |
|     256 | NOMETABELA                 | COLUNA2DATABELA             |
|       8 | NOMETABELA2                | COLUNADATABELA2             |
|     160 | NOMETABELA3                | COLUNADATABELA3             |
|     ... | ...                        | ...                         |