What size cake tin?

[shufflebum]

I need to make a cake for ds1 birthday to feed 12 children and 10 adults. I need the cake to be square/rectangular. Any idea of the size of tin I should use and then cake quantities?

An 8" square pan should be enough for 24 slices of 2.5" x 1". A 4 egg Madeira sponge would do the job:
4 Medium Eggs
8oz SR Four
8oz Caster Sugar
8oz Butter
4oz Plain Flour
Zest of 2 lemons
4tsp Lemon Juice

40-50 minutes Gas 3 (160 deg.C)

Brilliant thank you!
If it needs to be chocolate can I miss out the lemon juice and zest and use cocoa instead?

Shufflebottom, I think that a substitution of 1 desertspoonful of the plain flour for 1 desertspoonful of cocoa powder would be helpful. It may then be an idea to keep the citrus in there, but to substitute orange for the lemon.