You got to use XML to mod the size.
XML Is built into the game so go to the mod section on the main screen of the game
Once activated a new button should appear in the build menu
Select the cannon part and click the new button, a menu should pop up
A drop down bar should be at the top of the menu, go to the cannon section and find diameter, there you should be able to change the diameter, it's in percentages so you'll have to guess and check for a bit,
That should help!
@PapaWii Thank you! Now I'm able to use 152mm and 5mm.
You got to use XML to mod the size.
XML Is built into the game so go to the mod section on the main screen of the game
Once activated a new button should appear in the build menu
Select the cannon part and click the new button, a menu should pop up
A drop down bar should be at the top of the menu, go to the cannon section and find diameter, there you should be able to change the diameter, it's in percentages so you'll have to guess and check for a bit,
That should help!