Newbies here....Curious about this lifestyle

I know this may sound like a stupid question but if a wife is accustomed to a average size dick for a number of years how can she handle a larger one without being in pain.We are a mid 40's couple looking to expand our horizons with adding another male,preferably black, to have a threesome or to pleasure the wife while hubby watches.Hubby is around 6inches with normal girth and I have never had larger than that.Hubby wants me to find a much larger dick to have threesome with but i don't know if I can handle that much.Help!