Hello, and welcome to the site.
First, I doubt size matters like you think it does. There are many many aspects to pleasuring a woman that have nothing to do with penis size.
Second, if you aren't able to honestly communicate about how you each feel about your sex life together, then I assure you swinging is not going to work out well for you. Good communication is the foundation upon which swinging is built, and if you don't have it yet, then that's what you need to focus 100% of your efforts on, improving the communication.
Third, swinging and surprise are two words that should never be said together, so no, arranging something without her knowing is not a good idea. One, it goes back to the communication thing, since by their nature surprises don't involve two way communication, and second, doing something like you are suggesting is not showing much respect for your wife. If a couple are already experienced swingers then some sort of special surprise on something they have already proven they like and are into by their past actions, then that's one thing, but to hit someone cold with swinging, that's a good path to divorce.
Just talk to her, and keep talking, and the answers will come.