I would advise you to leave sufficient space for Windows 7 - it grows larger and larger as you use it. It seems to me that 30 gigabytes is ideal. (larger if you intend to install some other softwares) Ubuntu doesn't really need a dedicated partition to mount /boot; if you want to make everything as simple as possible, I would suggest:

1. /: A fresh installation of Ubuntu 12.04 takes approximately 5 gigabytes on /, so if you are not planning to get a lot of softwares then you don't need too much space allocated for it.




2. /swap: The common size of this partition = 0.5 * size of the memory, but I see you have some 16 gigabytes of RAM ... Ubuntu itself doesn't produce that much stuff, so just for fun, 2~4 gigabytes.




3. /home: theoritically this is where you should allocate the most space (But as gajdipajti says, you do not need a seperate partition. If you do not specify this during installation then these stuff would be put onto /) . However as you're saying, you prefer to put this on your external hdd. I advise otherwise. You may allocate some 4~8 gigabytes for /home on ssd, and create symlinks to link whatever you put on your external hdd.