tank size???

ive yet to find info on how to determine tank size for bags, and im a noob so could use some guidance. my truck is a 73 dodge crew cab and i will be using slam re8's. plan on running an engine driven compressor and was thinking either an 8g or 12g tank?

I like 8gal, its a great size because 5gal seems to run out immediately and 10-12gal takes a long time to fill, altho if you have the EDC it wont be that long, however if you have the 8gal it will fill even faster.

I'd run a 12 gallon with a EDC.