You can relate the diameter of the hole and the pin two different ways. Both of them depend on both parts (the one with the hole and the pin) to be in the same assembly.
Since you already have both parts modeled, and you undoubtedly want to locate the pin inside the hole, I recommend this first method.
****Method 1****
In the assembly, (1) peer variables for either the pin part or the hole part, whichever you want to be the DRIVING dimension. (2) Highlight the dimension that you want common between the pin and the hole by clicking on the row to the left of the variable column. (3) Right click and select copy.
(4) Next, left click on the other part, either the hole or pin depending on what you selected above in step 1. (5) Create a new variable name and press enter. (6) Right click in the value field and select Paste Link. (7) In the expression column of the DRIVEN dimension, type the name of the variable you created in step 5.
****Method 2****
Since the post is long, I'll just state that method two would be utilizing inter-part copies and including the diameter, with or without offset, of the dimension you want to be the same.
--Scott