I’ve flown both Qantas and Virgin Blue often enough, and have never had any problems with respect to my size. I’m 155cm/5′ tall and about a size 26 at the hips and 22 on top. Recently very few flights I’ve been on have been full, especially at off-peak times, like daytime on a weekday.
Qantas: I fit into the economy-class seats on Qantas’s 737s, and it’s not painful to have the armrests down but it is more comfortable to raise them. The seats are the same width as on their 767s. The armrests do go up on most seats, and the seating charts available on the Qantas website indicate which. The only exception is that the armrests on the window side are not raisable. I usually need a seatbelt extender, though you get the odd plane where the seatbelts are longer for some reason. The Qantas flight attendants have always been perfectly polite about getting me one when I ask. I have not flown on a Qantas Dash-8 but according to their website the seats are the same 17.5″ width as other planes.
Business class seats on Qantas are about 30% wider and have more legroom, but cost three times as much usually. There is no economy plus on Qantas domestic, although sometimes they fly a 747 between east and west coast to move the planes around and you can luck out and get a whole row to yourself on those!
JetStar flies Airbus 320 and 330-200s. I’ve never flown with them but I have heard from people who have that it’s much the same fitout and service as Qantas.
Virgin Blue: Seating is much the same as on Qantas on their 737s and 767s, and also on the new Embraer 170/190 jets. The armrests go up again except for the window side. Embraers are 2 x 2 seating, so there are no middle seat worries. Virgin Blue flight attendants have also been super nice about getting me a seatbelt extender.
Virgin Blue’s Premium economy seats are not wider than regular economy. They just have a kind of console/armrest thing where the middle seat it, and slightly more legroom.
For both Qantas and Virgin Blue, online check-in is available, and I highly recommend doing it as soon as it’s open, usually 24 hours in advance. Go for a row at the back of the plane, and an aisle seat is usually best as you can raise the outside armrest. They usually do seat assignment from front-to-back, so if the flight’s not full, chances are you will get an empty seat next to you. From what I’ve read from airline reps, they don’t have a compulsory two-seats-for-fatties policy as they know that nearly everyone who’s quite large buys two seats or goes business class anyway.
Rex is a regional airline and so flies turboprop Saab 340s. I couldn’t find any information on seat width, but other airlines with the same planes have 16″-17.5″ seat width listed on SeatGuru. The layout is also 1 x 2, so if you called to check whether the armrests went up, a seat in the single-seat row could work out for you.
Tiger - I’ve never flown with them. They’re a very budget airline – no business class, user-pays for everything. That’s about all I know.
Internationally, I’ve flown with British Airways on their 747s. Seat width is a standard 17.5″. The flight attendants were also very very nice and polite, and one on-the-ball fellow even brought me an extension without asking as I rolled my eyes upon seeing that the seatbelt was about 2cm too short. “Here you go, sweetie! Aren’t these seats just ridiculous?!” He also showed me the trick of putting the complimentary pillow on the tray table so my meal tray would sit flat, as my belly gets in the way of the tray table folding all the way down; and he also asked the person in front of me to put their seat back up so it was as un-uncomfortable as possible.
Again, online check-in is a good idea. Go to SeatGuru and check out the seat plans first. I was travlling with my boyfriend, and on most 747s there are a few outer rows at the back with only two seats in them. We picked one of those and while it’s a little bumpier down the back of the plane and you get people queuing for the toilet standing next to you – if you don’t mind that then those seats are a good choice.
British Airways Economy Plus seats are no wider than standard economy and may in fact feel smaller as they have the solid armrests that wedge you in.
I’ll be flying with Cathay soon, and I’ll be interested to try the new style of seating they have – it’s a “shell” seat and some people have said they’re terribly uncomfortable, complaining of too-thin padding on the seat and lack of leg-room as the seat slides forward instead of reclining, while others have loved them. I’m short enough that legroom is rarely an issue, and I’m well-padded myself, so we shall see how they are. They’re the same width as usual, apparently.