It's all in personal preference, but I wouldn't go any shorter than 26", and wouldn't go any smaller in diameter than a Magnum Sporter contour (not sure what the number would be, as each mfg uses a different numbering system). Personally, I like heavy barrels, and prefer the Remington Sendero/Varmint contour.
I have a #6 Krieger contour that is not fluted. Think it is called a heavy bull sporter contour. It is 26" long and .75" diameter at the muzzle. My 7 mag is heavy, but the weight absorbs some recoil and it shoots very well, so I don't mind carrying it.