The purpose of a double block and bleed assembly is to provide positive isolation which a means of determining if that isolation is holding. The purpose of the bleed is to test that isolation. It is not there to vent gas or liquid which is passing one part of the block therefore its size is relatively irrelevant IMO. If you have a bleed passing gas or liquid then you don't have a block and you should fix the block before continuing.
A DBB valve on the other hand is a different animal and a DBB valve does not provide the same level of isolation and ability to isolate items as it only blocks from one side or the other.
"KP-LOK Electric allows us to do our jobs better. Without it, we couldn't keep up!"
Better Switching power supply