Is there a way to blur an image in a surface without reducing its size? And also avoiding multisampling.

Most blurs mix the two ideas. First downscale, then perform a gaussian blur. You can skip the downscaling if you want, it's just there to improve performance. You'll need your surface to be part of a texture so it can be used as a shader input, and another texture to render to. The PostProcess sample in the SDK does this, as does the HDR Lighting sample for blooming.