Oh good question. Really its not so much the tank size as the fill rate of the fueling nozzle. Assuming a 25 gpm (90 ish lpm) commercial
pump rate, a 3/4" (21mm) pipe would vent air at about 4 feet per second (1.25 mps) assuming a tight fill connection with is not bad. This would move .05 CFS of air give or take. A longer vent more then a few feet (meter or two), might want to upsize to 25 mm ish.
Note, There will be blow back at the fill unless the vent is much larger as the air will vent at the easiest location, that is at the lowest mean static pressure drop, which is usually at the fill. The vent hose length adds to the pressure head
. So even with a 25mm vent you might still get venting at the fill port, Its just easier for the air to move that way with the low pressures during fill
Of course if the deck
fill connection is short and large then a small 1/2" (13mm) vent might be fine. Lots o varibles depending to fill routing and vent routing. Short vent pipes work better. From an engine
operating standpoint the 13 mm vent is plenty big enough.
If its a concern you could use two deck
fills, one as vent and one for fast filling. That way you don't have a large vent opening to flood the tank with a boarding wave.
Using a fill dip tube, that is a pipe extending down to say within 25-50 mm of the tank bottom would prevent back venting at the fill, But your going to want that bigger 21-25 mm vent hose then....