Assuming standard bi-polar motors: NEMA stepper motors have two important descriptions you will need to review for your application. 1. The physical size of the mounting face (NEMA 8, 11,17,23,34) all refer to the dimension of the front face. 2. The Force/current rating, which will in part determine the number of coil stacks used in the windings and so defines the height of the motor. (lower the impedance, higher the force, higher current requirement, more windings = bigger motor). Most of the standard motors are designed to 1.8 deg full step, but driven by a micro-stepping unit you can improve the resolution further.
I have chosen NEMA-8 for a focuser, with 0.14Nm - which is tiny, and is perfect for the job, but well over powered/engineered. A stepper motor from a CD player would have just about been enough.
I would expect that NEMA 17 would be a bit on the large size for your application, but you might need a higher force depending on your mount loads you are planning.
My 3d printer uses NEMA 17, but others work well on NEMA11.
I'm running NEMA 23, 4Nm for a CNC machine, it has over 300kg of force on the spindle to drive the axes.
You might also consider motors with build in position decoders - they do exist - but a bit pricey, and interfacing them to a raspberry pi might prove interesting. This would be the approach I would investigate if I were considering this project.
One final point. I would strongly recommend using a proper stepper motor micro-controller units which is able to drive higher voltages (recommend using 24v will do - all motors will tolerate this). These controllers avoid burning the steppers out by regulating the current flow allowed into the motor, and so get far more force/power for the motor size. These controllers also lower the power consumption with idling, which keeps them cool.