Most manufacturers give 3 month warranties to pro's and 1 year to personal/household buyers...but no difference really because if there is anything wrong with the sucker it will manifest itself pretty quickly...so from that standpoint you should be good getting whatever deal you think is good...and perhaps if in a store with a guy in it with any seasoning will give you good advice on what works for him and what he's heard from other users...could also get recommendations from tree trimmer contractors, they might be a good resource.
18" should do.
I've read stuff about people taking the 290 over the 291...more favorable "anecdotal" evidence of use FWIW...
I think Echo's have longer warranties if that is desired...
Anything 50cc + should suffice for general purpose needs...70cc + gets into Pro series, which (grunt!) is always fun if you can afford it...but they'll be heavier to tote...
Oh, and with the Dreamers and other Obamians out there...
http://www.miamiherald.com/news/weather/hurricane/article172862521.html...a good chainsaw might come in handy for other hurricane-relief needs!
ETA - PS: Here ya go, JF!
https://www.weaselzippers.us/356888-nun-with-a-chainsaw-goes-viral/Git-R-Done!