I've heard some wondering about how big the orcs will be....it appeared from some of the requests for extras that the orcs would be bigger than they were in the book.

Here's a quote from TheOneRing.net, from an extra who wen to auditions:

"The ad in the ‘Matamata Chronicle’ said "Official Lord of the Rings extras casting call," and said that they were looking for men and women under 5’8" in height. Filming to begin later this year, some of it in Hinuera, which is where the Hobbiton set is being built, not far from Matamata....

[much later]...Eventually it was our turn to be measured and have our photo taken, and then it was all over. Afterwards some people said they could see that on their application form they’d been marked down for some particular type, orc or hobbit....

[later still]...We hung round and saw a woman don a rubber head and shoulders mask and wave some sort of club or sword around for the benefit of the photographer. (She looked very competent and was rumoured to be someone in the Hamilton Sword and Shield club, or the Society for Creative Anachronism.) The onlookers argued about whether it was a Gollum-mask or an orc-mask. I’ve got no opinion, it could have been a Halloween mask from The Warehouse for all I knew. Nasty sallow snubnosed thing it was, but presumably such things are painted later. We weren’t asked to try on a mask. My guess is that if they asked you if you were claustrophobic, bad luck, you’ve been picked for an orc and can look forward to spending more time than you’d like encased in a hideous rubber mask, and none of your friends and family will recognise you in the movie. You hope. "

So, that seems promising. The orcs will hardly tower over the men of Gondor.