There's lots of packages available to increase/decrease bitmap size without pixelization. For myself I use PhotoZoom Pro but I believe that this very same spline fit engine comes built-in in Corel X6. I went from X3 to X6 so I can't speak for X4 & X5.
Whichever package you chose, they all use some variation of a spline fit which does indeed enlarge bitmaps with absolutely no pixelization. On extremely large enlargements, like a 3"x6" to 4'x8' which can be in the realm of reasonability, these tend to increase any blurriness and also tend to pixel bloom in areas of monotonic color.
The fundamental and inescapable fact when enlarging a bitmap is that information has to be generated out of the air to fill in the new pixels that must be added for the enlargement. This is pretty much always done by interpolating the value of the new pixel from the values of the pixels surrounding it. When those surrounding pixels, some of them or all of them, were themselves interpolated from other pixels, you tend to get into trouble. The number of engines and packages, each one dealing with this in its own way, are legion.