Now don't let's get the wrong idea here: roughing gouges are definitely not for producing rough work. Indeed, properly used they are extremely useful finishing tools as well. The primary use of the spindle roughing gouge is for reducing square cross-section spindle-turning blanks to cylinders ready for detailing with the spindle gouge or planing with a skew chisel.
NB: Do not under any circumstances try to use a roughing gouge in bowl turning...at any stage.
When looking at the grind on a spindle roughing gouge, you will see that the 'wings' of the tool are NOT ground back: the tool is ground straight across by rolling it side to side on the grinding rest without swinging it side to side at all. This is very important to its effective use. With a spindle roughing gouge correctly ground in this form, you will find that not only can it serve to remove large amounts of timber quickly, but it can also be used to plane the surface of the spindle blank to a high finish and can be used for the shaping of components with relatively gentle curves, such as the centre section of a stair baluster.
As to size, my advice would be, unless you're doing a lot of very small work, go for the largest size available. And OK, if really pressed to buy just one, I reckon it would be the 1-1/2" Taylor HS4, and I use this for everything from pen blanks and lace bobbins to newel posts - that's a versatile tool in my book!