The board size you use will also depend on the spacing of your studs. If using 2400 x 1200 board you need studs at 400 centres, if using 1800 x 900 then 450 centres. I've always used square edge board - you get square edges anyway when you cut pieces. Look for special plasterboard screws - they have a "bugle" head, not a countersunk head. They are also often phillips instead of pozidrive, so do use the correct tip!
If you are skimming it matters not whether you use taper or square edge - scrim tape the joints and get skimming. If you are DIYing are you going to have a go at skimming? It's not impossible for a DIYer to get a reasonable surface....