When I went, I used a Sony VAIO that was about the size of the 12" PB. (The last PC in my period with the dark side

)
I would definitely recommend a 12" PB. It's small enough that you'll never think twice about having it with you -- a big plus, I found, since I didn't need to plan out whether I was going to spend lots of time in the library or not.
I had an external monitor and keyboard and found that I pretty much never used them -- in the end, I wrote all my papers actually in the library. Now, that may be because (1) the law library had ethernet web connections and I had dial-up in my apt., (2) we had specific instructions to "use the books" on certain writing assignments and/or (3) if I was at home, I just wouldn't write the papers. Unless you're
really sure you want a monitor and keyboard, I'd wait -- you can always get them later.
Another reason that I went small was that I didn't take notes on my laptop (although the number of law students doing this clearly rose sharply just during my three years). If you find that you can, in fact, type while actively listening, you should consider taking notes on your Mac -- making your course outlines for exams will be much easier.
Also, it will keep you in the habit of writing a linear explanation of issues and answers -- while diagrams, arrows and pictures may help you understand the issues, you'll have to write them out on exams, especially if they have you type them. (From my 2nd year on, we used laptops to type exams, turning in a copy on a floppy disc. You may consider getting a USB floppy drive for this, or for sharing information in general.)
Keep in mind that the 12" PB has a pretty decent keyboard and an effective display as big as any that was called "14 inch" back in the day. I'd go with the 12".