This has much good advice on choosing the size of your tablet.
Just like with cameras, houses, and cars, no size is perfect, they all have their pros and cons. I generally go with: as small as is practical.
...some smaller publishers simply port their magazines to tablets by producing what is basically a facsimile of a print page. When shrunk, these pages are nearly impossible to read on smaller tablets, whether they have high-definition screens or not, without zooming in.
That is true. But for me, the same is true of the full-sized tablets, you generally have to zoom in on those too, so it doesn't matter. When you zoom in, magazine pages are perfectly readable on the iPad Mini, and in addition you get the wonderful lightness of that device. We waited for 2.5 years to get a lighter iPad, and it was worth the weight, I think the Mini is delightful to use. (And I don't have problems hitting the buttons, despite my large hands (span of 8.5 inches).
I like magazines in Zinio's format (PDF type pages, fixed layout, but live text, not just a scan), and I think it's a challenge to do it better, even if you design for tablets from the start. I like the rich pages, and I don't find it a hassle to zoom in on each column.
Below, a Zinio mag (Amateur Photographer) on iPad Mini:
Same as above, only zoomed in (this takes just a second, literally):