In truth, I don’t rightly know. It’s usually a gap of no more than about 3 m. There must be a prescribed legal maximum: similar to a mini roundabout being limited to 12 m diameter.
The one at Bournville station [picture above] certainly strikes me as confusingly big. (It's large here so that the signal covers sight-lines of lorries emerging from the green factory gate on the left, latterly disused).
A quick web-search revealed the law is silent on this point, but professional opinion's here.