according to the Giant literature on the Revive (the red one with 8spd's) the rear sprockets set is supposed to be
Shimano 11-32T 8-speed
however I've just checked mine to see if an 11/34 could be fit and found my largest sprocket to be 26 teeth.
Since Giant's website contains absolutely no contact info, my dealer will be hearing about this tomorrow.
If you were looking to change to a 34 tooth gear you already know that the gear range sucked. The 11-32 cassette is on 2004 models only. The earlier models came with the 12-26 which is not useful on any kind of hill. I changed our Revives to 11-34 and they are much improved with this ratio. Bottom line--your bike must be a 2003 and, yes, you will probably want to change it. However, you must also change the derailleur if you change the cassette. (I used Shimano Mega-Range cluster and matching derailleur--and lengthened the chain.)
I didn't actually count because I changed the chains on both (we have two) bikes to Sram. I bought three chains and it seems that I ended up somewhere around one and one third standard length chains on each bike.