you'll actually get used to whatever size you have. most 12" 13.3" and 14.1" (wide)screens will all have wxga (1280x768 or 1280x800) and most 15.4" too (corporate ones often have higher, latitude/thinkpad). i think the 12" on my 700m is fine. i actually do most stuff on it (and i'm on it now) because the screen is much nicer than my xps's (i wish i could see other people's xps screens for a comparison to see if i should get it replaced). i actually write most of my papers, browse the internet, light gaming, and use it at airports because it's a much more convenient size (i use my xps for gaming, cad, excessive multitasking (you know when you need like 5 windows all visible when you're prigramming?), watching movies (it's bigger than my tv, so i might as well)).
i would say 13.3" is too small if its your main computer. if it is that, and you do want to do things like play games and watch movies, i think 14"+ is a better bet. this is just because movies on too small a screen isn't that good and also because systems with dedicated graphics dont come in small sizes (for cheap, since the xps m1210 is of those exceptions)
basically what urchin was saying about pixels is right. if they all have the same resolution but different sizes, the bigger screens are not going to look as sharp. and get a higher resolution if at all possible. 15.4" will often come in wsxga. yeah, and speakers on most laptops sucks. use headphones when possible, and definitely when gaming or watching movies.
portability is definitely a key issue too. if you're going to be moving around a lot with it, get a smaller one, probably 14". i think it's the hp dv1000 series is a pretty nice compact size, though i dunno if its still around. go to a bestbuy or compusa or microcenter or any of those computer stores and check out the sizes. there is even variance within the ones with the same screen size. i had a toshiba that was 15.4" that had a rather large trim around the lcd making it bigger than it needed to be.