The path you where the you define the UNC share shouldnt really matter. In response to your question about the partition size I would not imagine you needing more than 600 meg allocated for this repository. Typically the repositories are not more than about 300 meg. The size will depend on the number and types of packages that are in the repository. Just set this up as a UNC type repository and make sure the creditials you provide has read , write, modify, create access to the folder you specify.
I hope this helps.