I initially ordered a Medium; everything I already have is a medium and fits well.
When I received it I spent the first ten minutes trying to work out where the rest of the waist strap was, but ended up realising that was all I was getting! When measured with a ruler, the waist strap measured in at 15 inches! (Just to remind you, this is advertised as an ADULT medium). At full stretch, I struggled to get the waist strap around a standard Premier League football.
I returned and exchanged for an XL which is still a very snug, bordering tight fit. I have a 32" waist, so if you're larger than that then you'll either be very uncomfy, or need to look for another guard.
I tried a size L Adidas Pro Guard which seemed to fit very well, if not a bit too big, so it may just be this one item which is affected by absurd sizing.
Other than the sizing, it's a good, sturdy product. Just make sure you order XL unless you're a tiny child!!