.68" calibre is the standard for paintball
.50" has picked up for both splatmaster .50 'low impact' play for younger players (and adults to join them) and for rental sites that may convert or startup at .50"
However - even though .68" is the standard, paintballs are encapsulated liquid, and aren't precise, have various grades, made in a few factories so you will find differences between brand / grade / batch / environmental conditions by fractions of an inch - and when propelled down the barrel at 300fps the liquid ball will deform as it blows down the barrel
Most guns/barrels will shoot most paintballs, but in extremes may be smaller or larger. For absolute efficiency and fine tuning you can get barrels and sizing inserts in slightly different sizes with ranges such as 0.671" to 0.691". A paintball could be 1/100 of an inch bigger or smaller then standard
The etek 5 is for .68" paintball, takes autococker threaded barrels and has a vast range of after market barrel options
For now just use it and unless you get bizarre inconsistencies then just use it as is for a while and play