2GB will do (my 2006 has 2gb and runs Lightroom 3 and PS CS5 at the same time). You should aim for 4GB though as this will be much faster.
Do you want speed or size? Speed means going for an expensive solid-state drive but sacrifising size, whereas size means you can get more memory at a standard speed. As you are going for a few months I would advise you go for size and get a 500GB HD or a 30GB HD.
My MacBook has a 13" display, sometimes, but not often I would like bigger. I would say that 13" is suitable for you, It makes the laptop lighter as well.
You will need these if you are going to be importing images from camera to computer.
You may also need to consider the spacing between them if you need to use a 3G dongle or other accessoires at the same time, such as a mouse, USB memory, etc etc.
Not specfic to each laptop model but size of screen. get a good one, I use a neopreane cover that absorbs all shock from knocks and bangs that would otherwise break a laptop, and as you are travelling this is an essential piece of kit.
You are travelling, will you always have access to power? make sure you can get additional batteries at a decent price, you will need them.