Yes indeed your capacity does matter but when does one just buy one LifePo4 battery? Once you see the benefits of them, if you did buy just one, how long do you think it will be before you buy two? If you do buy at least a pair, which pair on the market will take less than 100A?
One thing on the back of my mind is that if my batteries are at their minimum, it will take about 12+ hours to fully charge. Knowing the RV industry, how many converters can charge at the full capacity for that long? I'm not talking specs, talking about reality. Would I recommend a converter that could deliver more than you would use? In a heartbeat.
Would I recommend swapping a converter out when swapping to LiFePo4 batteries and go larger? Yes. If on a budget, get the batteries first, then converter later. The stock converter will be just fine. Won't full charge them but most [if not all] car manufacturers do not recommend to charge your car full unless you are going on a long trip.
One thing for certain, people really need to look at what their needs/wants are and not just go with someone else's opinion that might be totally different. The more you go down the LiFePo4 battery hole, the more questions you will have. If you like making your own decisions and like research, eventually, it will all make sense. If you like others to make decisions for your, I am certain there are a few business' that you could pay to get good advice and sell you some good products.