If you do not install other apps then a standard install of BT5 R2 will take approx 14 GB, so you could have your partition of about 16 GB.
I have Dual boot Win7 and BT 5 R2. I am accessing my data from windows and also from Linux easily. Here is my arrangement
My BT partition is 23 GB. I have installed many other applications like k3b, kget, ktorrent, vmware, opera ... etc.
I create a separate NTFS partition of 54 GB to store data from Windows and as well as from Linux so i can easily access my data from both the OSes.
As we can not have HOME folder on NTFS so i moved Documents, Videos, Music, Downloads etc folders to that NTFS partition.
This way you can use your BT partion only to install BT and other progams and keep your data on other partition.
I am also a newbie for Linux, i do not know if this arrangement is good or not, but it is working well for me.