Again the info you need about projectile size will be found in the reference sticky.  
There is a lot of useful information on the stickies that we have collected over the years we have had this board..
Computing bore size:To compute the proper bore diameter for a given object to be used as a cannon projectile: 
Measure the diameter of your perspective projectile.  Divide diameter by 39.   Multiply the result by 40.
Bore Size of commonly used items for cannon projectiles:A guide to common can sizes 
Can sizesA bowling ball gun with proper windage would need a minimum bore of 8.815 inch.
The circumference of a ball shall not be more than 27.002 inches nor less than 26.704 inches.  27.002 xi .31831 = 8.595 maximum diameter of a bowling .  (8.595/39)x40= 8.815 inches 
http://www.topendsports.com/sport/tenpin/balls.htmThe Pressure tank need to make the expansion chamber for a bowling ball mortar for is the 300 CF, DOT 3AA2400 made by Taylor Wharton. 
http://www.taylorwharton.com/assets/base/doc/products/cylinders/TW1102_StandardHighPressure.pdf  A golf ball gun with proper windage would need a minimum bore of 1.723 inch.
The diameter of a golf ball is 1.68 inch. .  (1.68/39)x40=1.723
http://www.learnaboutgolf.com/beginner/balltypes.htmlA popcan mortar with proper windage needs a bore of 2.667.  
A Diet Coke can is 2.6 inches in diameter.  (2.6/39)x40=2.667
Dimensions and weight of cannon ball sinkers.
http://www.barlowstackle.com/Do-It-Cannon-Ball-Sinker-Molds-P303.aspxDimensions of Pool Balls
All balls must be composed of cast phenolic resin plastic and measure 2 ¼ (+.005) inches [5.715 cm (+ .127 mm)] in diameter and weigh 5 ½ to 6 oz [156 to 170 gms]. 
http://wpa-pool.com/index.asp?content=rules_spec A pool ball cannon or mortar should be (2.255/39) 40 = 2.313 inch bore.
Dimension of a Tennis ball.
...the acceptable range for ball diameter (2.575 inches to 2.700 inches respectively for a Type 2 ball). The accuracy of calibration is 1/-0.005 inches.  
http://www.itftennis.com/technical/research/lab/balls/size.asp  measurement conversionsConvert: 
http://www.joshmadison.com/software/convert/ handy little free ware program to convert all sorts of units of measurement