Yeah, since you're so light, you probably want a board in the 140 range, give or take. You might get away with 143'ish if it's a flexible board like a park board.
Your budget is also a constraint... pretty much you're looking at 2012 snowboards on discount, and often times the smaller sizes sell out. You just have to make a list and see what's available...
Since you're in the East Coast, you might want to look into boards with an edge tech (Frostbyte for Burton, Griptech for Arbor, Vario or Never Summer, Magnetraction for GNU).
Lilfoots' lists are usually pretty good. GNU and Never Summer out of that list is probably considered 'higher end' because I think those boards are Sintered Base (considered better than Extruded base, but it won't matter for a beginner so much).
Also consider Roxy snowboards and Arbor snowboards. The Arbor Cadence might be good also.