Recipes usually call for large eggs. The American Egg Board has a website that specifies substitutions for different sizes (i.e. 4 mediums in place of three large, etc.).
As far as egg/dairy ratios, I can't answer you specifically. If I wanted to answer that question for myself, I would collect a number of recipes for custard or quiche fillings and develop a ratio from them.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan