Buying big.....bigger..... or the biggest is not something we are going to recommend you when it comes to raku kilns. Of course the space you have in a raku kiln is used less effective compared to an electric kiln. Unloading just after kiln has just been shut off at a 1000 °C requires space to move around with your raku tongs in the kiln. Very much unlike an electric kiln that you can stow with ware as far as you can and unload it only when it has cooled off again. So you do have to look with a different view to the size of a raku kiln than when buying an electric kiln.
Still, do not by a big raku kiln because " Then I can fire al lot of pieces at the same time....".
This does not work.

The smaller the ware is, the shorter period you have between the unloading of the kiln and start of the reduction.
If you fire too many pieces at the same time the unloading may take too long. By the time you have the last ware out of the kiln, the first pieces will probably be cooled off too much. And if the ware is too cold it will not pick up the smoke from the reduction.
On the other hand if you put those last pieces too quickly in the reduction chamber without sufficient time for the temperature shock, they will not have the crackled surface that you may want them too have.

So here's our recommendation for choosing your personal raku kiln:

Choose a kiln size that can contain 2 or 3 pieces of your work but not any bigger. When necessary fire multiple times. This way you can give each piece the attention that it needs for an optimum result.