Great idea. I have done something similar with my Nestabilities.
I have 7"x9" 3-ring binders that use 6¼"x8½" page protectors (available at Staples, Office Depot) & store embossed die cuts of all sizes for each Nestablitlies set in a page protector.
I use them in the same way you do to help with card design. I have also saved the "frames" which are helpful for determining best size & die style to use when I decide to die cut around stamped images.
This size binder is not cheap so I watch for them at Goodwill for 99¢ and stockpile them. I use them to store completed cards, card fronts, stamped images to be colored, etc. by card types (e.g. birthday, sympathy, wedding, masculine, different holidays, a whole bookshelf full!)
Using your example, I'm planning to start one for all cards sizes. Thanks for a very helpful, time saving idea.