select
  *
from
  <table>
where
  size< 300000 

select
  filename
, sum(size)
from
  <table>
group by
  filename
having
  sum(size)< 300000 

CREATE PROCEDURE XXX (
    S DOUBLE PRECISION)
RETURNS (
    ID INTEGER,
    FILENAME VARCHAR ( 200 ),
    FILESIZE DOUBLE PRECISION)
AS
  declare variable ns DOUBLE PRECISION;
begin
  ns= 0 ;
  for select
         id
       , filename
       , filesize
      from
         folder
      into
         :id
       , :filename
       , :filesize
  do begin
    if (filesize is not null) then
      ns=ns+filesize;
    if (ns<s) then
      SUSPEND;
    else
      exit;
  end
end