6" or > girth is a good fit for regular magnums, with no slippage.
6.25" or > girth can work for magnumXLs, likewise very little to no slippage with that girth, if you stay good and hard.
best to have a good fit and not just get the XLs unless you really need them
for comfort or the regular magnums are breaking on you because of size issues.
ask someone else around here if your girth is bigger than 6.25".