Find if file exists and report size back - custom inventory 7.5

I feel like I've searched and searched and searched and havent found the answer I am looking for.

My end goal is to find the size of a specific file, c:\Temp\Access.mdb

I created a custom data class and set the attributes but I am now stuck on how I can populate that information. I would like the task to run daily and allow me to see which of these files are approaching the 20mb threshold of this file.

If someone has step-by-step instructions or a very good how-to then i would be much in debt to you.