Omg this is my thread. (: I own a library of manga.
I wouldn't shy away from series just because they are long, if you truly enjoy it it's nice to show your support for the mangaka. But if anything, just buy the arc you like.
But for series that aren't long-winded: Cardcaptor Sakura ~ a great intro magical girls manga with a fun story plot and not too long (12 volumes), plus the art gives you intro into the wonderful world of CLAMP Anything by CLAMP ~ I should have started with this, srsly anything by CLAMP is brilliantly written and beautifully drawn (and not too long-winded) Eureka Seven ~ even if you've seen the anime, the manga is amazing and only 6 volumes, also the art doesn't shy away from the anime art so they look very legit, love it
Crap, I can't think of any other ones right now but I might add more later...