A woman called me today to say that the person who was supposed to be doing her mother's wedding cake just had emergency surgery and will be unable to do the cake. The wedding is just 4 days away. I've never done a wedding cake, but since they are looking for something simple given the time constraints, I said that I would do it. The cake needs to feed about 200. I am looking for any advice on cake sizes, as well as some simple decorating ideas. Thanks!

200 is not a small cake. Are you comfortable stacking cakes? If not (or if, in the interest of time and saving stress you just don't want to go there), go buy or rent a pretty stand, bake whatever sizes work on the stand and make sheet cakes to make up the difference. Keep the decorating simple and maybe even ask them to have the florist supply flowers for YOU to put on each layer. (don't let them do it)

If I had something sprung on me in 4 days, that would be my sanity-saving plan.

What shape of pan will you be doing? Square or round? Or something else? And how many tiers does the bride want the cake to have?

Square - 10, 12, and 14 will give you 220.
Square - 12 and 16 will give you exactly 200

Round - 8, 10, 12, and 14 will give you 196.

I just am not a fan of rounds going up by just 2" in size so my suggestion would be to do 14, 10, 6 rounds and have an 8" on each side OR use kitchen sheet to make up the difference.
If you have sq pans then use a 14" sq on the bottom w/12, 9 or 10 & 6 rounds. That would be nice too.
Good luck. It won't be all that bad if you have the time to devote to baking and icing it nice & smooth.
What do you have for support? Any SPS? If not, use Wilton's 'hidden pillars' w/a plate on top - very similar to the SPS. You will only need 8 pillars and 2 plates. The 6"er can be supported w/drink straws.

Thank for the suggestions. After looking at my time constraints (I teach full-time), I think I'm going to go with the 6-10-14 rounds with a sheet cake for serving. At this point, they are desperate to have a cake for Saturday so hopefully this idea will work for them.