It depends on the content. For fixed layout content (comics or other color/design laden reading) bigger is better, and that's mostly what I read on my 12.9" iPad Pro. It's fantastic. This content tends to have 'spreads' which in a physical book would be facing pages. IMO 11" is not big enough for this, so I'll probably never get an iPad of that size.
For re-flowable text content, even an 11" screen presents an area larger than any physical book page one is likely to encounter in a novel, and I find it awkward. iPad mini is best size for this, with iPhone a close second. I like using scrolling mode on both. If I needed larger text then maybe 11" would be nice (but not as versatile as 12.9").
When I do find myself wanting to read long form text on my iPad Pro, I like to use stage manager to shrink the app window to about iPad mini size, as I don't like 2 column mode for this type of reading. It is usually attached to magic keyboard, hence landscape orientation, and I can use the trackpad or cursor keys to turn pages without touching the display. But this usually involves having it on some desk or table, which is not where I prefer to read.
For more technical books, using full screen with 2 columns is great as illustrations and diagrams can be viewed along with discussion without flipping pages as you would need to on a small screen. (Unfortunately on Kindle platform many of these are in print replica format, which does not allow 2 pages to display, no matter how large the display is. Fortunately it is possible to convert them to PDF and use some other app to read them on iPad or Mac.)
And sometimes Split Screen is useful so you can have some reference material or note app or Maps on one side while you are reading on the other. Again 12.9" wins for that.