I fly often with my 6lb chi and my cat. We use the delta sherpa LARGE carrier, which is plenty of room for them to be comfortable. Can you get the bigger one? 8 hours is a long time to be cramped and not be able to stand, IMO. Although neither of them use the bathroom, during layovers, I do have a pee pad and we go to the family restroom and I let them walk around. Like I said, they don't ever use it tho. I keep a pee pad in the carrier because if they have to go bad enough, they will.. I personally take them out of the carrier in the airport. As long as she's on a lead and within reach..I've never had an issue. We try to find an abandoned corner or something, where there aren't many people and especially no kids. I have a portable water bottle and bowl which collapse easily. You can find them at any petstore just about. I don't withhold food or water. For 8 hours, your pup could eat before the flight, and maybe take a few treats to have during the flight or on the layover..even a little ziplock of her food would be good if you think she'd eat it. I'd always offer water. No behavioral changes at all. Keep calm, don't be stressed because she will pick up on it, be cool
One last piece of advice, I'd take her carrier out now, if you haven't already, just to let her get comfy with it and hopefully she'll use it as her own little secure space. I took Miya's out a month before our big flight to Japan and she napped in it every day, so come time for the big flight, I know she was comfortable.