Coils as long as 25 mm have been produced. In general, the higher the magnetic field, the shorter the useful magnetic length. Also the more narrow the bore, the shorter the useful magnetic field. A perfect probe can never perform better than the magnet homogeneity allows. Many vendors use long coil lengths, which make magnet to magnet variations more important.
Some vendors optimize probe coil lengths based on each magnet.
If you are in the market to buy an instrument, this can be one of the potential reasons why the sales demonstrations can be better than the performance you get on delivery. Length of the room temperature shims' active region When the main magnetic field is long enough, the probe designer still needs to be concerned with the useful area over which the RT shims operate. If the main field is long enough and the shims don't operate with the necessary purity over the coil's length, the probe's performance will be suboptimal. RF Homogeneity Solenoid coils can have good RF homogeneity over long lengths. However, most superconducting magnet probes must use some type of Helmholtz coils.