The sophistication determines how hard the floor is, and the size determines how long it will take to RS gold finish. Both are dependent on the party boss. Everyone chooses complexity 6 because you get the most XP that way. The dimensions (the number of rooms there are), again, is dependent on what the party boss chooses. When there's 5 people on your party, expect it to be set on big and require a very long time to finish.
When you finish a floor for your very first time you receive double XP. The next time you just get half. This is known as stature, but you can get double XP again if you click on the reset button. Thus, everyone does all the flooring they have unlocked once Like flooring 1 - 27, clicks the reset button, does floors 1 - 29, (because they lvled dungeoneering by finishing dungeons), clicks reset again, and it keeps going like this. Try not to perish as it lowers the XP you receive.
If you're F2P some of the rooms are members only doors. They aren't unlockable so don't worry about them. If you need any help about the puzzles, ask the folks in your party or buy OSRS gold check Rs Wikia. They've an awesome puzzle manual.