Careful about buying on ebay for a hard board as a lot of them are cheap far eastern factory productions.
Typical problems are warped decks, poor shaping and inadequate cloth / resin as this latter thing is where the cost lies. Inadequate cloth / resin makes the board weak and can lead to all sorts of nasty problems such as dings, crushed / split decks and even total breakage.
Not saying that every board is bad, but from what I've seen advertised a hell of a lot look dodgy, brands you've never heard of (try googling the brand), websites with no shop address or contact numbers only mobile numbers or web forms. Look on here, there are a couple of people who've posted looking for spares for some of these boards as the company has disappeared and they can't find spares (read specific fin fitments) anyware.
Try asking on
www.a1surf.com 's bulletin board for brand advice but generally ebay is not a good place to look for a board unless you're looking for a specific 2nd hand board from a reputable shaper.
I'd try a proper surf shop such as:
Beachbeat Surf Boards
www.beachbeatsurfboards.co.uk
or
Tunnel Vision
http://www.tvsurfshop.co.uk
(Both Cornwall).
You'll pay a little more but get a better board. A cheap board will cost you £'s more when you constantly have to have it repaired or have to trash it entirely prematurely.
As for board size, ask a shop expert as opinions vary and the suitability of a board has more to it design wise, than just mere length. A whole host of factors including rail shape, tail shape, thickness, width, fin set up, all contribute or detract from stability / directional stability and its only with the right combination that you'll truly find a board that makes you happy.
Also, consider whether you want a foamy, combination board (foam deck + rigid underside) or a hard board. If in doubt, again consult a reputable surfshop - best found from word of mouth as some just want to sell you a board rather than offer real independant advice. Find a good one though and you'll get all the help you need.