The forthcoming revision of Traffic Signs Manual Chapter 7 will have examples of the use of these Y-arrows, but no dimensioned drawings.
At Buchanan Computing, we have implemented them in our SignPlot software by scaling them off the drawing in TSRGD. We make the arrows 36 sw high, and the 2-headed lane bifurcation arrow 24.5 sw wide. We follow as closely as possible the same rules for vertical and horizontal spacing as for other dedicated lane directional signs.
Since 2016 TSRGD came out I’ve been looking out for these signs on the road network, but have only so far noted one (on the northbound split between the M6 and M42(S)).