You’re big compared to any group of people, and if your fiance is happy with it and says not to worry, then don’t worry. From the sound of things, she might not like your insecurity over something that she is perfectly happy with. Also, don’t worry about comparing yourself to her former lovers. Women are notoriously bad judges of penis size, so those guys probably weren’t much larger than you, if at all.
Most penis surveys find the average/median length to be about 5.5”-6.5” BPEL, and average girth is around 4.75”. You’re substantially bigger on both counts. Porn is evil. Ever notice how the guys in porn rarely get their dicks all the way in? Something to think about.
As for maintaining the EQ, kegels :) I also recommend staying in good cardiovascular shape, avoiding junk food, and maybe taking ginseng or L Arginine.