I do not think it is a problem in their marriage. This same thing exists in my marriage and it is a wonderful marriage spanning more than 44 years. It is simply a communication problem which can affect your sex and personal life but is not symptomatic of a troubled marriage.
When I started working, I was asked to use simpler words because the employees were not understanding what I was saying and many stopped reading after the first few sentences. I got a program that I could set for whatever school grade level I wanted to and it would change my words for me. 10th grade level seemed to include all employees.
Good communication is making sure that your audience understands what you are saying. You are using unnecessary fancy words for reasons only known to you. I can ask my wife to pass me the spherical orange sphere in the refrigerator or simply pass me the orange. That is not dumbing it down. That is using plain language when plain language will suffice. Sometimes using "fancy" words is just showing off. I have very high IQ, gave speeches and seminars for a living in a former life and I do not act like an intellectual just because I can. I was lucky in that my parents never let me skip grades or once mentioned IQ. They did get me into a class for gifted children in a private high school on a scholarship, but I really did not even know what IQ meant when I was 13. I grew up normal due to my parents and being in school with others like me which did not make me feel like I was special. In fact, I was a jock who joined the Army to experience combat. I always viewed myself as an alpha jock. I made the all star teams in football and baseball and have trophies in Volleyball, basketball and bowling. I did well in sports due to my ability to learn to play every position on the team and memorize every position in football. I also got good grades, but that was easy to do, not like sports.
I am in a business where few people have a college degree. I deal with warehouse and truck driving staff. I have been told since my Army days that I have a gift of fitting into whatever situation and group of people I am in. I can curse and trash talk with the best of them. I can fight if that is what it takes. My face is scared from my fights, football and combat. I was never a geek. Yet, I am in a geek profession; IT. I did not start in IT but I made sure that I ended there. I like to be down to earth when I deal with people and can up my game when I am sitting down to talk to the Chairman of a fortune 500 company. I have written many technical magazine articles and co-authored a reference book which was popular because I have a knack for explaining things in plain language that most peope can understand.
To me, being understood by a lot of people trumps showing off my vocabulary. I grew up a son of parents who spoke broken English and my vocabulary was terrible when I entered the business world. I had a Brooklyn accent and all through school and life I have had to look up words. However, I learned to speak on the same level as my audience no matter if I was giving a speech to a group of businessmen or highly technical people. That is good communication.
My wife used to tell me that our friends did not like sitting next to me because I spoke on a level above them. They also could tell by the way I spoke and what I said, that I was intelligent. Add what my wife calls my forceful alpha male personality and physical size, and that adds up to not having a lot of social friends. So I became careful of how I spoke and what I said. I stopped questioning things people said that I knew were wrong and instead did what everyone else did and nod my head in acceptance. I still relapse at times but the problem is us, not them.
I am not reading into your post what others do about it not being about the way you speak and instead a marital issue. I know how your wife feels. Often I do not communicate well with my wife who went to a trade high school instead of academic. It is my problem, not hers and has nothing to do with our marriage of 44 years which has quite frankly has been great. I could have said inestimable, but I did not.
I do not think your wife is being unreasonable or pedantic. She just feels like you are coming across as didactic/pretentious. As someone in the international transportation business, I have to blend in and I have learned that it is not dumbing down my language, but rather effective communicating. It does not bother me because I was raised in government assisted housing and never met any kid with a dad with a college education until I was 13. I learned english on the mean streets of the lower east side of Manhattan and not in some prep school. I am comfortable in any environment. Just as I speak Italian in Italy, Spanish in Mexico and French in France, I view language as speaking the tongue of the people I am with. I even speak, or used to speak, latin but never actually did so out of the classroom as that would seem too pretentious to me.
Your wife is right. Finger widths would be a better choice of words to use even though that is not a very good way to measure something since it is subjective to the width of your fingers. I would have told you to give me the information in inches, not the breadth of your particular fingers. That is not only an unnecessary way of describing it but also not a very accurate way either.
P.S. - I know that I tend to be rambling and verbose, but that is the writer in me.
Also, my wife does not say anything anymore and that results in her not understanding what I am asking or saying. That is not a good thing.