I don't have any Magnum stories (for obvious reasons), but I used to get reactions occasionally when I bought my brand, Contempo Exotica. (For those who don't know, they're condoms for small guys like me, and are 15% shorter and 6% narrower than average prophylactics, according to the manufacturer).
90% of the time, the clerks were totally professional and never seemed to even notice. If I got a reaction at all, it was usually that the clerk would avoid eye contact with me and not chat as much as usual. A few times, I got stifled giggle. When they moved them behind the counter at the pharmacy where I got them, a pharmacist once asked if I was sure, because "these aren't the normal sized condoms, sir".
And one late night purchase, when the place was really empty got: "Really? Wow." from the girl behind the counter.
These days, I avoid potentially awkward situations and just order them online.