Scratchbuilt parts done to scale can be done with the aid of accurate and detailed drawings of the prototype or if none are available, through a careful study of photographs of the prototype.
For example, the railings of a Type VII Wintergarten appear to be made from two different size of tube/bar stock. The top railing is larger than the mid rail and all the verticals, with two exceptions, appear to be the same size as the mid horizontal railing. (I say "appears" since this is based on studying the photos.)
My take is that the larger bar stock is approximately 2.5 to 3 inches in diameter and the smaller is somewhere between 1.75 to 2 inches. (this from memory....so please beware!) These numbers were based on observing several crewmembers hands resting on the top railing and noting how far around the railing there hands would go.
I also took a look at an engineering materials handbook from the 1930s and found out what was then being used within industry for safety railings....more as a sanity check than anything else.
-hope that helps!