As you have read, it's a big place. But the entire property being quoted includes the 4 theme parks, at least 20 resorts, some of which are huge, a shopping village and all kinds of highways and major roads.
Park size varies by park and wikipedia is your best source for this but if you want to context it to a place you have been, let us know what other theme attractions you have been to. While they all vary in size, they all feel about the same- Sea World, Universal Studios........but Magic Kingdom probably has the most attractions per square foot I am guessing. It's definitely a 2 or 3 day park to see and do it all.
WDW is best done with lots of planning. Even veterans plan their visit around park hours, fireworks, extra magic hours, dining reservations, etc. Get yourself a planning book like the Birnbaum and educate yourself on what these parks have to offer and what you think your family will enjoy. There are You Tube video on almost every attraction so if you aren't sure go there ( if you don't mind a spoiler).
I find the planning is a great way to get excited and tons of fun.