Erm size is fairly irrelevant
i dont think its impossible to have a digital portfolio AND a printed one. its good to show you have some reasonable amount of digital know-how when you show off your portfolio, so i think there is definatly room for a PDF presentation on disc or something like that to show off a few digital images (and show off your photoshop skills)
it is certainly essencial to have decent prints avaliable as well. The best portfolios are those with few prints of good quality
ten is your absolute maxium, try and get a range of ideas and also think about which images go well together - so you may have four small images mounted onto one piece of card in a sequence, as well as a few large sized good images
Always mount on white or black, thick card
get a leather portfolio, either A3 or A4 depending on what you think you will need and basically have ten pieces of card with either sequences of images, or just plain images on their own
your portfolio should change regularly, as you will always have better prints coming through the more you improve
