estimate how many guns you currently have add in everything on your realistic wish list then add 10. remember that safe companies over estimate the ammount that will relistically fit in a safe by double. so if they claim 19 it will prolly only hold 8 or 9. they estimate it based on the smallest guns without slings or optics.
i like keeping things other than guns in the safe as well.
another thing to consider is where to put it in your home. some safes require reinforced flooring. even though a floor may hold it up right now over years it can do real structural damage. just depends on the weight. my safe is stored in the basement.
just some things to consider
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent