You can use windows built in backup software to image the partition or use a free program like easeus To-Do Backup.
http://www.todo-backup.com/products/home/free-backup-software.htm
An image makes an exact copy/clone of the partition. Normally you do not have the option to restore single files from an image, instead it will restore the entire partition. A standard backup will backup individual files and folders andt thus you can restore individual files and folder. This is what you normally want for your data partition.
As for partition size, according to Microsoft -
16 GB available hard disk space (32-bit) or 20 GB (64-bit) . So, you will need that amount of space for the OS + additional space for programs you want to install. It's better to overestimate than underestimate as it can prove to be difficult to resize the partitions later. A 40/60 split may not be bad, depending on the size of the drive.