The last sentence of advice in this thread is spot on: use a lab with a high end Nikon or Imacon scanner (which comes very close to quality of drum scan at much lower cost). I worked for twenty years (until end of 2015) at a small pro photo lab and had many of my slides developed there and scanned like this. BTW Nikon film scanners are no longer made so they will be increasingly harder to find.
Assuming you have a sharp slide to begin with, meaning camera was on a tripod with decent lens and sharp focus, you can expect a great scan from iso 50 or 100 slide to make a print as large as 30x45 inches (apologies for using American inches if you are in a country that uses metric). If it is an extremely sharp slide, perhaps bigger. I have one print in my home from a Nikon scan of Ektar 100 (negative, not slide) that is 40x60 inches. If you stand with your face right up to it you can see grain and a bit of softness, but from any normal viewing distance (e.g. six feet or more) it looks amazing.
Film is meant to be printed and looks great in print. It is NOT meant to be viewed digitally (which is where digital cameras excel, though of course they print well also). So your scan will likely look quite grainy on the screen, but that does not mean it will look that way when you print it.
Someone has also correctly stated that scanning too high does nothing but enlarge the film grains themselves and does not add sharpness. It actually has the opposite effect - it makes the picture look worse. A good lab will know the optimum scan level. At our lab (JonesPhoto.com) we found 40mb to be the best for 35m film.
One final note. Slide film is very high contrast with Velvia being exceptionally severe. This means it works best in low contrast scenes, such as overcast days or clear days in the soft light of sunset or sunrise. In high contrast scenes, such as under bright sun, you will not get the same range of tones and detail you would get with digital (especially a RAW file) or even with print film.
