An interesting comment by Game of Thrones co-producer Vince Gerardis: the Battle of the Blackwater will last 16 minutes of the penultimate episode of Season 2.
This is an interesting figure. The Battle of Philippi, the largest previous attempt by HBO to depict a massive battle sequence on a regular series (on Rome), lasted for nine and a half minutes. The opening D-Day sequence in Saving Private Ryan lasted about fifteen minutes. The opening battle in Gladiator lasted just under ten minutes. The battle for control of Babylon 5 in the Severed Dreams episode of that series lasted six minutes.
Sixteen minutes - more than a quarter of the episode - is an impressive amount. I wouldn't be expecting anything on the lines of a major movie action sequence, but it'll be interesting to see if HBO are opening the purse strings a little more this season (not they didn't have a big budget last year, but they certainly weren't getting the most bang for their buck).
The episode in question - Blackwater - is written by George R.R. Martin and directed by Neil Marshall (The Descent, Centurion) and should air (barring delays) on 27 May in the USA and 28 May in the UK.
In the same interview, Gerardis also says that HBO are holding the three (soon to be four) Dunk 'n' Egg prequel novellas in reserve for another two years or so. This suggests that HBO have a gameplan for using the novellas to flesh out their Game of Thrones content. Pure speculation, but it's possible they are thinking of using them as a spin-off series (Dunk and Egg wandering the Seven Kingdoms is actually a solid premise for an ongoing TV series), for between-season straight-to-DVD movies, or as stopgaps if they have to delay the main series for whatever reason (akin to what Starz did with the Spartacus: Gods of the Arena mini-series when the main series had to be delayed for a year).
Amendment: According to Westeros.org, Gerardis's comments were mistranslated (the interview was originally in Portuguese). Gerardis stated that the battle took up a sizable portion of the episode and is 'fantastic' but is still being produced with TV budgetary and time constraints and wishes they could have done more. He also stated that the prequel novellas are being looked at and hopefully in a couple of years HBO will be able to do something with them, but for the moment are concentrating on the main series.