Indy, what is the minimum size sewing machine for leather?
Sorry missed the question.
I'm sure you can do some leather on a regular sewing machine, but thicker materials require an industrial machine with what they call "triple feed".
Sometimes called a "walking foot" machine.
Normal machines push the material through using a series of serrated teeth under the material. These are called "drop feed" machines. because the teeth drop down at the end of the feed and index a specific amount to start another stitch. Most home machines and most tailoring machines are drop-feed.
A "walking foot" machine adds a second presser foot on top of the material that "walks" with the drop feed mechanism pushing the material through from both sides. It's a much more positive feed and necessary for thick materials. They are much more complex because the presser feed has to be timed exactly with the drop feed - so they had a series of shafts that come from the bottom to the top side to feed the walking foot.
A triple feed machine has a needle that also moves with the presser foot - which makes them "triple feed".
Another factor for working on thick material is the power of the motor. It takes a lot of torque to push the needle through 3-4 layers of leather. Most of the time home machines don't have powerful enough motors or enough structural rigidity to keep from flexing under the force.
Most of the modern walking foot machines are copies of the old Singer 111 series machines which can go back to the 1930's - in fact many companies still call them 111's.
An industrial machine will come on a workbench type base usually with a clutch motor drive. Although professionals learn to use a clutch motor, I found for an amateur like me a servo motor was necessary.
I have a 80 year old Singer 111-w153 and a 30 year old German made Pfaff 145 which are very similar machines. The Pfaff has a few more features, but doesn't really sew any better.