I'm sure you know this, but size is SUCH a personal preference that depends entirely on how an individual wants to use their device(s), because we all use our devices very differently. And each size of iOS device--iPhone or iPad--has its unique balance of portability vs. screen real estate and features. The difficulty in making a decision really comes in deciding exactly how you want to use your device(s), and what is important to you--eg. price, one-handed use, easy pocketabilty, big screen for movies and tv, shopping in bed, e-reading on a bus, font size, big keyboard typing, 3D touch, sketching, hand writing--the list goes on and on. No one device covers everything, yet some choose one device to cover as many of their needs as possible (probably the 5.5" iphone). Others use 2, 3, or even 4 devices to specialize in specific needs or cover a greater range of needs. The disadvantage with using more devices is, of course, higher price and the fact that you have to keep track of multiple devices. And to complicate matters there are tons of third party solutions to cover many of the things Apple devices don't provide. Yet with all this variety, it's still very likely you won't find the absolute perfect solution to all your needs, depending on how unique your needs are. There will be compromise. It's just a matter of finding the least amount of compromise for you.
As for me, I am currently using an iPad Pro 12.9" in addition to my iPhone 5s. I've tried other combinations including two different iPad Mini models and an iPhone 6s+, and so far this has been the winning combination for my needs. The 6s+ was way too big as a phone for me and it was an even worse tablet with its relatively small screen, 16:9 aspect ratio, and especially its lack of support for either portrait or landscape mode in many apps, requiring me to constantly pick up the "tablet" while it was docked to turn it. I found the Mini a good casual tablet (although I preferred it to be a TAD smaller so I could palm it comfortably or put it in a pocket--tad smaller, not 6s+ size which is 60% smaller), but as a creative professional I opted for the 12.9" iPP with the Apple Pencil. It has been a GREAT tablet for creativity, productivity, and consumption in all the contexts that matter to me. I was concerned I wouldn't be able to use it casually because of its size, but I found with the right stands and arm mounts in the right places around the home, it can even be used casually in place of an iPad Mini--even better because I don't have to hold it. The few scenarios in which it is too big, such as perhaps reading on a bus, happens so rarely for me that I'm fine using my 4" phone in those cases. Those are the compromises I'm ok with. It's all about personal preference.
