The solution is relatively simple, and I thought I'd write it up here for a place to copy syntax each time.
APEX users also have a handy table to verify this against (apex_application_files). Well, a synonym/view that ultimately maps to the core table wwv_flow_file_objects$.
It contains a doc_size column, which is no doubt evaluated at some point during upload of the file. For blobs in your own table you can use the supplied package DBMS_LOB to get the length of the file in bytes.
Multiply that by 3 orders of magnitude and you have it roughly in kilobytes - a digestable figure for most files dealt with these days (calc_size).
Add some pretty formatting and you have how Windows Explorer shows file sizes (calc_fmt).
Inside an APEX report you might consider the format mask "999G999G999G999G990" on 'calc_size' - without any trailing suffix so you can still order the results.
select id, filename ,doc_size ,round(dbms_lob.getlength(blob_content)/1000,2) calc_size ,to_char(dbms_lob.getlength(blob_content)/1000,'999,999')||' kb' calc_fmt ,apex_util.filesize_mask(dbms_lob.getlength(blob_content)) size_mask from apex_application_files order by doc_size
/* UPDATE 2014-07-11 */
Carsten (@czarski) pointed out on twitter that an API that's available that returns a rounded off relative size with the appropriate trailing KB/MB/GB/TB.
It's not shown in the current 4.2 documentation - but I think it's probably just been missed rather than it not being supported. The APEX_UTIL package is pretty large should probably be broken up so little nuggets like this don't get missed.
He also mentioned a "FILESIZE" format mask, which isn't shown in the format popup (you just type it in) and also behaves differently to the API. Values under 1MB returned just numbers to 3 decimal places, and a 10MB file was labeled as 10KB - this was on 188.8.131.52
The only downside to the API formatting is as per calc_fmt, the ordering is impaired - by ordering on this column you will get results like:
So in my sample results run in APEX SQL Workshop I can see tiny files to progressively larger files - up to an APEX application export of about 1.6 mb & a PDF over 3mb.
|Filenames redacted to protect the guilty|
Note, this view can't be queried from SQL Developer (or your tool of choice) since the current workspace is part of the view. Martin has an interesting post to get around this sort of thing by defining an APEX session using PL/SQL.