Essentially I have to make two riders. The Don who rides a horse and the servant who rides a donkey. So there are going to be two horse skeletons - Dinko explains that in the Unicorn project of the book. What I'd like to comment is the wire used. I like the most the type of wire shown on the picture it is a 0.8 mm diameter stainless steel wire. The roll is 50 meters long, so it should be enough to work with on a pretty good number of projects. This type of wire is a bit sturdier than the one sold in arts and crafts suppliers' shops. It can be easily obtained at any hardware stores for the approximate price of 9 $. I prefer this type of wire because for me it is easy enough to work with and at the same time sturdy enough to be used on bigger projects (long necks for dragons, sines for leviathans) too.
The next thing I'd like to discuss is the usage of wire, when making complex (well, fairly complex) skeletons for the creatures. It is good to always make sure that the piece of wire you are working with is long enough. It is visible on the right hand side of the picture how I left the tail part of the skeleton long. This is because I started with a really long piece of wire. On the next picture is the cut tail.
Sure it is possible to adopt the 'scientific approach and make exact measurements. And if you are inclined to do that, go ahead and do it. As for my own work I don't use that approach. I find it too time consuming and fun ruining.
The size of the skeletons you make depends on several factors: how big you want the creature to be, how big is your oven (you will have to bake the creature at some point), how much polymer clay you are willing to spend for this project, where will you place the creature once it is baked.
It is probably possible to make exact calculations but with time and experience one is able to make very quick rough estimations and combine all the mentioned factors. So I am going to stick with the rough estimates and comparisons. But for those who have a scientific approach the best I can do is take a picture of the Don Qixote's ride next to a six inch ruler. I would lake to apologize in advance for the obscure terminology I am going to use, which will include the 'scientific' terms: smaller, bigger, more, a bit, a bit less, small, big, tall, fat, bony and alike. I believe that the picture with the rules must be a good reference point for all the 'scientific' terminology.
Please note that in this project the horse skeleton has two main differences from the one shown in the Unicorn project in the book. Variation 1 - the space between the front and the hind legs is significantly smaller. The aim is to create an impression that the horse is tall and bony. A starved specimen of his kind. Variation 2 - the tin foil used on the body is significantly less than in the Unicorn project, again in order to create the impression that it is a tall, skinny horse - not the type you'd expect to win races or to stampede the enemies in battle.