I went the big pants option as a) in case of c section, you'll not want anything too tight or at risk of creasing near site entrance and b) by that stage of pregnancy and afterwards I'd said a fond farewell to small, non-cotton pants.
Maternity mats are approx 75cm square absorbent mats: they put them under you when in hospital to catch well whatever is needed. As suggested, you might want to start sleeping on one (under a sheet) in case of water starting to break. You can wash a sheet, you won't want to wash the mattress.
Re cotton wool: a couple of bags will be fine I'm sure. Some mums never do the cotton wool and water thing on new bums. Some compromise and use water wipes. In likelihood, you'll start with best intentions with the gently warmed water and using a fresh cotton ball for each wipe of baby's bum, and then relax into the world of baby wipes.
I've got a couple of packs of size 1. We have just used pampers on the basis if it's not broke, don't fix it. I must get our subscription on amazon prime set up... I took the view that it's easy to send family members/ other halves to pick up x brand of nappies in whichever size if and when needed. That said, if you over order, and baby grows out of the size quickly, I'm sure friends will thank you for the excess, or food banks etc.
The nappies per day is difficult to gauge as every baby will be different. Our first loved nothing more than to use a clean nappy so each change took 2 effectively. I think I used to work on rough calculation to be safe of 8 per day when I was packing changing bag for the day.