..
Sheets come in 8 feet by 4 feet(about 3.5 square yards). So if you don't own a van you're as well to get it delivered...i know in Cork must of them will deliver for free, so check the merchants in your area.
3.5 square yards means 3 sheets if you can use all of them, relative to the cost, consider getting 4 sheets rather than having to use all the cuts off the bare 3. They are only a few quid each...like less than 20.
You would be as well to be honest to lift the toilet and sink pedestal out, lay the boards, tile over them and then put the fittings back in...
The biggest thing other than that really is the fact that you need to square every 4 inches in every direction...so try and have the use of 2 drills...one for drilling, and one for driving the screws in...