Barrel/Paint sizes

Barrel Bore size
ACI zero gravity .692"
Check It .690"
32 Degrees .689"
DYE  .690"
DYE boomstick .688"
Jacko Infinity .691"
Lapco Autospirit .687"
Lapco Big Shot .689"
PMI Perfect .691"
SP All American - grey .686"
SP All American - black .689"
SP All American - dark grey .692"

Paintball sizes: Since no paintball is exactly like another, these would be the average of the diameter of the paintballs...

Small: .688" and below
Medium: .688" to .690"
large: .690" to .693"

Paintball Type: Size:
RP All-Star Small (very small)
Diablo Hellfire Small (very small)
Proball Platnum Small 
Proball Small
Diablo Inferno Small
RP Marbellizer Small
RP Big Ball Medium
RP Premium Medium
RP Premium Gold Medium
Zap Medium
Diablo Blaze Medium/Large
Nelson Large
PMS First Choice Large