I have a Pocketbook 360, which has a 5" screen. Yes, it's small - especially when I compare it to my Irex DR800SG.
In FBreader, which is my primary reading software on the PB360, I've got my font set at 30 pitch, which is pretty big (but then, I like big fonts, what with my pitifully bad eyes). That gives me 18 lines of text on the screen. Yeah, not so much for displaying lots of text.
But.
I do not notice having to turn the pages all that often. Honest! Once I am in the book, it just flows. Plus, I have FBreader set up to only refresh the screen every six page turns (you can set it higher or lower), so the pages turn very quickly.
The bennies of the smaller reader is the weight. Instead of always trying to find a way for my hand to be comfortable supporting a book, be it a paperback or a hard cover, AND be able to turn a page with the other hand, I merely press a button. Big difference.
Yeah, a big screen is nice. But, having a smaller device just makes it easy to settle in for a long session of reading (and I read for 4-5+ hours at a time). My reader fits comfortably in one hand, with a page-turning button readily available to either thumb. Or, I can prop it on the sofa arm and use my forefinger. Regardless, the ability to just use a button to turn the page makes you forget that you ARE turning so many pages. If that makes any sense...
So, in a nutshell, yes, you will turn the page a lot more with a 5" screen (especially if you have a larger font size). But, if the novel is good, you really won't notice it.
Hope that helps.
Marilyn