I went for the 33 & 46ltr. The 33 on exhaust side evens things up regarding the width of the bike. It does make the bike wide but I think it's manageable. I also have a 52ltr top box (looks massive but I don't think its out of place on the bike.
I went for the Givi Trekker style but if I was buying now I might go for the Outback ones, they are slightly larger or even the Dolomiti style, although I haven't seen them in the flesh yet. (other manufactures are available)
If you can, go have a look at some in the flesh, try and decide what you will be carrying and how much space you need, but don't go too big or you will end up taking things you don't need!! After 5 years of touring we now carry about 1/2 of what we did on our first trip.
There's loads out there, do some research, find out what you want then check prices, you can get good discounts if you look around and don't rush in to buy.
Good luck.