For a start, 3½" is big enough to work quite acceptably well. Indeed even 3" can be made to do something useful. It's when you get down to the guys with 2" penises that penetration becomes exceedingly difficult.
But if you can't do without sex from someone bigger, then go for someone bigger. But be warned, someone bigger won't necessarily be any better and someone who's good now may not be be good later in your marriage and you may not be able to enjoy it with anybody, big or small, later in your marriage. Marriage is about more than the size of a penis and certainly more than about sex.