@AmberPrayfrock welcome to matrix inventory! This is common in the lumber, pipe, and apparel industries just to name a few. If you find an add-on for 100 that handles it, please let me know.
One option is to use lot's for tracking quantities by sub-category (ex 8' vs 6' 2x4's). That would also allow you to track cost for each group separately. However, it's going to get ugly in a hurry. To address board feet, you would need to add an UDF at the lot level to add this as an attribute of each lot and then get into custom reporting to work with the resulting values for stock, sales, and purchase transactions.
We discussed something along this line with a company which milled logs into lumber and then used that lumber in the manufacture of pallets. Some high-value cuts of lumber they produced they sold wholesale. In our solution design (Sage 500 rather than 100 but the same principles apply), the customer agreed to set up SKU's for each cut and classify each board by SKU. In other words, a 2 x 4 x 8 was one SKU and a 2 x 4 x 10 was another SKU. If a board measured 9', it was considered a 2 x 4 x 8. For board feet in this case, it becomes an attribute of the SKU. This is a lot easier to manage than at the lot level.
In the end, these are the types of opportunities we walk away from early as they we feel they not a good fit for any Sage ERP product that we are familiar with.