For me it would be about 10 small, 35 medium, 35 large, 20 xl. never sold xxl or above.
Now, I'm going to make some horrific over generalisations now. No one be offended!
Your average US market is going to skew the sizes larger. If you sell on the internet and not to shops, that's going to skew the size larger (as, on average, the average internet customer is larger than the average high street customer). If you sell shirts that have a geek or gaming appeal, that's going to skew the sizes larger. If you sell a unisex shirt with crossgender appeal, you'll skew the sizes smaller. A large European market will skew the sizes smaller. Phrase and text based comedy tshirts tend to sell to a larger size of demographic than arty design shirts.
All sweeping generalisation yes, but always held up for me.