Normal screen sizes vary from around 21.5in to 28in, and 24in is currently the sweet spot, if you've already got one of this size then you've just saved yourself some $$$$$.
You have to ask yourself this question, are you going to be streaming movies on this new PC regularly, if so you'll probably be better off investing in a better television and streaming to that than a new monitor, stick to a monitor of 24in for everyday computing and you'll not go far wrong.
Remember, the bigger the screen and the higher the resolution means that your graphics card has a lot more work to do. Besides that you'll have to sit further away from the PC, not much point in getting a 48in monitor if you have to watch it from 30ft away is there?.
