I have a 13-inch MacBook, and I do extensive score transcription and editing work on it using Sibelius. I have never had any problems with the screen size. In fairness, I have no experience using a larger screen so I have no means of comparison, but the smaller screen has always worked smoothly for me – I’ve never had a reason to wish for anything bigger. Even when I’m tweaking layouts, with the pages shrunk down to fit on the screen, I can still see the score perfectly well. And I love the portability of the smaller machine – especially when I’m traveling!
As for the mouse pad – well, I’m used to it so it works fine. I do use an external number pad plugged into the USB port so that I can take advantage of the program’s inputting shortcuts. I find that inputting in this way goes very fast, even faster than using a full music keyboard.