Assuming you want to put the OS on the C drive and the data on the D drive, then size the C drive to hold the OS and your applications and the D drive for the rest. Depending on how many applications you have, 60 to 100GB is probably adequate for the C drive.
There's no law that says you NEED two parititions - you could also use just one large partition for everything. The biggest reason to use a separate C: partition for the OS is to reduce the size of an image backup of the OS. If you don't plan to do image backups of the OS then you may not need separate partitions at all.
If you're very careful about how you distribute things, you can use partitions to improve performance. But given your question I think it's probably simpler for you to use as few partitions as possible.