as a student going into uni this year i have also frequently looked up this topic, and in my opinion the best size is 13 ".
why?
netbooks, which i have tried (and refunded at myer for gift cards) are complete rubbish. their atom processor can barely handle essential tasks such as booting up, as well as running 2 microsoft words. in essence, this type of laptop is useless, the fact that the screen is tiny means that you need to scroll down frequently in almost every aspect of daily use. the battery life however was appealing, but another thing is they tend to have rubbish exhaust fans, particularly the eeepc, which is reported to be super noisy ( i can vouch for that, i returned mine, wayyyy too loud)
i also have a 17 inch macbook pro which i took on some occasions to school, but barely, because basically it is a lump of metal and it is huge and heavy. this is completely impractical for use in university.
the problem with 15 inch is that while they are at a good size for gaming, movies, and work, many companies sell off rubbish components in them because normal computer components sometimes fit in laptops, like the amd athlon x2 processor, the problem here is that they use a lot of power leaving a low battery life. however if you are careful 15 inch is pretty good.
why do i think 13 inch is the best? at this size components need to be specifically designed to fit into the chassis of the laptop, meaning they have been more thorougly considered, also, they are very portable, and are sligtly bigger thanthose dvd players.
the laptop i am after personally is the 13 inch macbook pro