They won't be too large, but for that size room m60's would be more than adequate. Also the m80's are a 4 ohm design so some receivers could have issues driving them.
As far as plasma goes, I recommend you look for a 1080p model. The pixel resolution should be 1920x1080. Pretty much all the 1080i and below models use 1366x768 resolution. If the budget allows, you might want to consider an LCD. The contrast ratios are significantly better than they used to be and you won't have to worry about burn in or the brightness fading over time.
I have 2 LCDs. One is 1366x768 (32" Syntax) and the other is 1920x1080 (46" Samsung) and for 1080i material - which is deinterlaced by the monitor since LCD,Plasma etc are progressive by nature - the picture is noticeably sharper on the 46".
