If you set them to large, the receiver will send the sub-bass to them as well. I don't recommend that with the M5s. As great as the M5s are, if you have your receiver's gain set anywhere near 0, the bass from the M5s will distort during peaks. Even if you had M100s I wouldn't do it because it will tax your Yammie.

So leave them as small. It is going to sound great and you'll be kicking yourself for not doing this earlier!

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated