It takes different amount of wind force and wind speed to blow down different sizes of trees.
For a small tree the wind blowing at 30 to 50 MPH could easily blow down the small tree and if the tree is diseased or decayed that can also factor into it blowing down with less wind speed.
To be able to blow down most bigger trees though that are healthy and in good condition without rot then the wind speed usually has to be around 70 to 100 MPH or more in wind speed to be able to knock down a tree or uproot it.
If you have large trees near your home they really should be topped or removed or at least checked for diseases and decay because if wind blows strong enough it can topple the tree and sometimes land on your home and cause major damage or even death or injury to someone inside the home.