Not sure, as I have only mixed sizes once .... this year.
My daughter put a selection of different eggs under a broody peekin. They all hatched and so far so good. One chick looks like a Rhodie cross Hyline. This chick is about 3 times the size of the smallest chick, which is a pure peekin.
They must be around 6 weeks now and mum seems to be keeping the big chick in order with several sharp pecks .... he is a cockerel and quite cheeky.
I suspect it is best for the eggs to be from breeds of a similar size but probably a bit of bad luck too.