Remember that over long term use Windows installs and the program files folder with SQL Server in will more than quadruple in size from their initial install. On a production system you would want to allocate as much dedicated physical storage as possible to the databases, however on a development system the size of the platform is likely to vastly outweigh the requirements for data.
I would suggest an SSD (or two in raid 1 for durability if budget allows) dedicated for the database server to use and make sure that the OS partition is larger 120GB or more in size unless you're using server core (200GB suggested if installing tools so you have some free space afterwards).
If you can't dedicate storage to the databases and this is a dev environment then have a single large partition - the purposes of separating the OS from the other data is to protect the OS from the disk being filled up and provide some very weak security against path issues (and allow segregated disk quotas for file-server users). By using a single volume you reduce the chance that either the data or the OS will fill the volume.