This has been discussed over and over here. I'll be brief (and a little blunt, sorry).
Generally speaking, if you want large prints you are doing everything to make your chances of high quality unobtainable.
I would say if you want to go to 20 inches, you ought to use a drum scanner. You can get drum scans very cheap over there, from Tim Parkin. Pay a little more for more megabyes, as much as you can afford. If you are going for a smaller print, you can use a dedicated film scanner. A 20 mb file is a useless file for printing (the ones I use are over a Gig), good only for snapshot size and for posting on the web.
The film you are using is not good for scanning. You should be using something in the 100 ISO range, like Ilford Delta or TMax. Move down from the Portra 400 to the 160, or use Ektacolor for your negs.
Next, buy a larger camera, or rather, a camera that uses larger film. And a tripod.
Lenny