Using a ballistics calculator, I came up with a 185mm artillery piece with the following performance. Why that? Because few/no real guns have the caliber, and I wanted something between 152/155mm and 203mm. When not obtained via the calculator, results are extrapolated from the S-23, the closest real life equivalent, with some enhancements like faster loading and lighter weight to simulate better technology:
Maximum range: 27-41 km, depending on ammunition.
Rate of fire: 2 rounds/minute
Average shell weight: 70-75 kilograms
Approximate Mass: 13,700 kg
The artillery piece is usually self-propelled on a tracked mount, but towed variants do exist. It tends to serve as a corps/army level weapon whose primary goal (and largest shell portion) is counter-battery and other missions where range is more important than size, although it can do anything a big gun can. Advanced users have developed nuclear shells for it, and the usual conventional ammunition types (regular HE, cluster, etc…) have been made.