Roy, mate, your cock is well above average but I wouldn't have thought that Magnum's would be right for your size. I'm bigger than you yet I can wear regular condoms with only mild discomfort. Condoms are supposed to be tight fitting and I'd suggest that if you're uncomfortable with regular condoms you may either be:
a, bigger than 5" thick - try re-measuring! or,
b, putting them on wrong!
Try experimenting but my biggest advice would be that if the condom ain't snug don't even try to have penetraive sex with it or you could end up with problems!
Another tip is to try They Fit condoms, which can be ordered online and are 'made to measure'!
Good luck!