Well, 900p (so I'm assuming a resolution of 1440 x 900) is still pretty sharp, even on on a 17" screen. Trust me, text becomes really small on a 17" 1920 x 1080 display.
Actually I think most of these "900p" displays are more like 1600x900 or something like that. They seem to be taking over for the older 1680x1050 screens.
As far as text size goes, that's going to be personal opinion. My 17'' laptop has a 1920x1200 screen and I don't find the text to be all that small. My best suggestion is you go to a local Best Buy or another place that has laptops on display and mess around with what they have and see what you like.
Dell does not make it clear but this is what I ordered.
17.3” HD+ (900p) Bright LED Display with TrueLife™ and Camera
Even with 1080p can you not scale to any size you want? Would scaling make text less clear because it is not the "native" resolution?
With the 1080p screen you can reduce the resolution to whatever you want all the way down to 800x600, but the same goes for the 900p screen it just can't scale up. And yes, reducing the resolution from "native" reduces image clarity and things tend to get distorted.