So basically all I want to know if I get this right:
The extra spatial dimensions in string theory and are required to be curled up and very tiny to explain a) only three perceivable spatial dimensions and b) the relative extraordinary weakness of gravity.
Now with M theory where the three spatial dimensions could be a 3-brane and to which open strings stick to and closed loop strings (gravitons) don't, this isn't required anymore. Three spatial dimensions come from the 3-brane to which all particle strings are limited (no curled up extra dimensions). Gravity is actually massively stronger but appears weak because most gravitons radiate away from our universe (no need for tininess/curvature of extra dimensions).
So we can possibly have 11 flat dimensions. Sounds about right?