A 12" round sponge will feed 61 people, a 9" round sponge will serve 32 if you cut into standard 1"x2" portions. Because some of the pieces will come from the edges they will be a bit small (the downside of round cakes!) it's better to have slightly more servings than people, iyswim.
Aesthetically, a 9" cake is quite big for a top tier and can look a bit odd because the decorations will be a bit disproportionate. Personally, I'd do a 6" tier on top, just to make it a bit more balanced.
Recipe wise, madeira cakes tend to be very dry and stodgy so I use a victoria sponge recipe with a little bit extra flour to firm it up - regular victoria sponge isn't strong enough to cope with icing and tiers - and all of my customers say it's lovely.
So, for a 12" round cake of 2.5"depth you'd need 12 eggs, 24oz butter & sugar and 28 oz sr flour plus a slug of vanilla extract. I have to say, I bake the really big cakes in 2 layers because they take hours to cook in one layer, plus they can hump in the middle which is a pita.
A 9" cake needs 8 eggs, 16 oz butter & sugar and 18oz sr flour, and vanilla extract.
A 6" cake needs 3 eggs, 6oz butter & sugar and 7oz sr flour plus vanilla essence.
Make them as you would a vicky sponge - cream butter and sugar, add 1 egg & tablespoon flour at a time, then rest of flour & vanilla extract.
I bake at 140 degrees, 6" needs about 40 mins, 9" needs a good hour to hour and a half, 12" (if you do it in 2 layers) needs an eye keeping on it, but around 40 - 50 mins.
If you want advice on how to assemble tiers etc, shout.
You will need to buy 6"/9" cake cards for the upper tiers though and wooden or plastic dowels (you'll need 3 or 4, depending on their length) plus a 15" cake drum for the 12" cake to sit on, just so you know what to order!