If i resize the window where i've rendered a textured mesh it showes the mesh blured, why?

Your rendered surface is being stretched onto your window, which accounts for the blur. You have to resize your back buffers when the window size changes.

Change D3DPRESENT_PARAMETERS::BackBufferWidth and D3DPRESENT_PARAMETERS::BackBufferHeight to the correct window client area size, then call Device::Reset(). I'm assuming you know how to use Reset. Let me know if you need help.