The 6" ones (base/paperwhite Kindle, Kobo Clara HD, etc) aren't that far off. My Clara HD is the same height as an n2DS XL and much thiner, just about an inch wider.
A Kobo mini looks to be roughly the same height as a small DSi even
(though I'd personally recommend something with a frontlight)
DSlibris is the best eBook reader for DS in my experience and actually quite good, but the DS just isn't a great device for reading on... The screens are just too small/low resolution (and that's from someone who hates large screens) and eInk is just much more pleasant to read on. I find 6" about perfect, though again I like small screens, many people prefer larger.
It at least doesn't appear to have any kind of page limit, tried it with a 1.5M word (~10k dslibris pages, I got bored halfway through at 5k as I don't think it can jump to chapter) book and it works fine, just took a couple minutes to load.