The ideal size is usually suggested as a metre square, but if you can manage a bit bigger and you have enough stuff to fill it, then go for it, remembering that the heaps go down quickly, and your volume includes depth, when you are considering heat retention. Also, what you can manage might depend on your height for reaching to the back.
If you have a row of bins, then they each keep eachother warm in the row (but you might find that metal gets quite cold at the ends - maybe insulate it with wood). Be sure to design them to make it easy to turn one into the next from time to time. It's hefty work, which you should really do a couple of times a year. I couldn't manage that, but my grandson loves turning the heaps and makes a point of doing all three in little more than an hour. He's young, strong and fit, so be realistic about what you can manage. I had originally planned a pair of compost heaps in such a way they could be turned with a tractor front fork loader. It didn't work though as at the time of year it needed to be done, the backwards and forwards repeat motion just chewed up the ground horribly. So we scrapped those and build a row of three. I have heard of people turning their heaps using a rotavator, but that hasn't worked well for us.
I actually don't mix my animal cleanings with the vegetable waste, but compost the two separately. This is so I have rich FYM and milder compost for different purposes. We also collect the used straw from the sheep field shelters every couple of years, mince it with a rotavator and use that on the outside beds - grows wonderful spuds!