I've been asked a couple of very good questions about chick igloos in cold brooding.
Remember, the igloo part is the mesh circlet that surrounds the chicks sleeping inside the brooder. Around the igloo is the stuffing that insulates the nest.
So to size: there's a very easy way to work out the right dimensions of this inner igloo for any number of birds. I do it with pen and paper — I draw an oval about the size of a chick, and keep adding bunched ovals until the number is the same as chicks I want to brood. A circle around the whole bunch is the size of the igloo.
I calculate that 15 standard-size chicks at 2 days of age will want an igloo about 20-22cm in diameter and 8cm high. If your igloo is 10cm high then add bulk to the floor and the chicks will be cosier.
It's best not to copy my exact dimensions as chicks vary. The main point to keep in mind is that the igloo needs to be only large enough to fit all the chicks when sleeping together, all touching. As they grow it needs to grow. So the best way to do this is to keep the igloo without a roof so it's only a mesh circlet, and make that circlet adjustable.
What I've done is to cut two strips of guttering mesh which are fixed to the box doorway. These are then overlapped to make an inner circlet of the right size (which is 100% adjustable). To stop chicks getting between the overlap I pin the inner flap to the outer one with a simple wire clip (the wire ends always poke out into the space of the box, not into the igloo where chicks' eyes can be injured).
Below is a loose diagram of what I mean, seen from above. The inner circle is the igloo; the outer rectangle is the box; the door is hanging open at the bottom.
The igloo in this version has no mesh lid attached so there's nothing in the way of adjusting the circlet. However over the top of the whole box (including the circlet) is a larger mesh lid which I can open and shut to add or remove stuffing and adjust everything. The blanket (or sack-cloth) sits on top of this entire lid.
But I just want to add one observation: brooding 15-20 chicks is far, far better and easier under a hen. In fact I haven't used the cold brooder this season at all, because I haven't had many chicks. Cold brooding really comes into its own when you have 30 or more chicks, no broody hens and want to reduce power bills.
I hope this helps. Apologies for my site's insufficient comments display... I'm working on that. :-)