As tubes mentioned, if you have the space to accommodate floor standing mains, my preference would be for them as opposed to bookshelf speakers. Having said that, I would consider the M60s instead of the M50s as I don't think the difference between the M22s and M50s would be worth the money, whereas the jump to the M60s would warrant it.
The room dimension-wise from what you noted should be plenty of room for larger mains, so again, if you can afford it and you like floor standers, I'd suggest them.
One other option if funds are a consideration, you may want to go with M22s or M3s for mains for now, then when you move from 5.1 to 7.1 (assuming you do), you can always move the 22s or 3s to surround or rear surround duty and replace them with larger mains.... -TD