Managed to delete orginal post !!!

Airbox size is

500mm long x 200mm high x 150mm deep with 3" outlet.

How important is it tune for a certain frequency,I hadn't considered this at all,I just have 3" ducting coming from airbox to nosecone

15l capacity doesn't sound to bad.

Although it is hard to predict on which frequencies the airbox helps to resonate.
Rules of thumbs give a broad range and CFD analysis is difficult and also not definate.

At the end it you want the biggest volume that you can store in the engine compartement.

IMHO

Not quite....

You want as big as you can go, without the risk of the air in the box being heated if its not being changed frequently enough. 15l is mighty big, so you may find you actually warm the air entering the engine... Just a thought!

As a rule of thumb plenum/airbox size would be 1.5 to 2 times the engine displacement.
A good guide here http://www.amsperformance.com/pdfs/intakemani.pdf