The London Pattern Book defines the curvatures and widths of gouges. Thus each is labelled with a pair of numbers. The first number is the sweep, the curvature and the second number is the size, the width, in millimeters. Major manufacturers and retailers often have a LPB chart in their website (eg Pfeil, Ashley-Iles, etc).
Since very shallow cuts are being made to texture the surface in what I suspect is soft wood, you can push the gouge by hand for control. At the shaping stage in a carving, it's common to use a carver's mallet (they come in different sizes and weights.)
In Post#1, I'll estimate the gouge as a 5/35. The edge is better than that of a new razor blade.
Depending on source (shop around), shouldn't be much more than $50.00.