I may be preaching to the choir - but these costumes came about somehow, yes? Does the company have a costume shop manager? At my ballet company, it is that person's job to inventory costumes (as well as a myriad of other tasks). If not and it falls on you, I can tell you what our shop manager does.
1. Create dressing lists for each ballet
2. Organize costumes by ballet on the rack
3. Small accessories (gloves, pins, removable decor, jewelry) in diddy bags next to each appropriate costume
4. Larger accessories organized by type, per ballet, in cardboard boxes (hats, shoes, etc)
5. Create giant "pods" of safety-pinned muslin to completely cover the costumes in the warehouse (grouped by ballet)
6. Label the pods with the ballet title, label boxes with ballet title and type of accessory. (If necessary, number the pods "pod 1 of 4" and same with the boxes. Also note the number of boxes on the pod label.)
During either dress rehearsal or performances, photos are taken. The shop manager saves the photos in her electronic archive along with dressing lists and show paperwork. Based on those photos and dressing lists, she pulls from ballets if needed in the creation of a new work.
I will say, because I've been with the same company for 12 years, I have a pretty good memory of what we have in stock, even though I have nothing to do with costumes/wardrobe. If this is solely on you, get as familiar as you can with the costumes. If you can do all of the above, you will be familiar enough with it all to recall items you have in stock, and based on your paperwork, know exactly where to pull the items from. When creating the new ballet, be sure when building your dressing list that you say where pulled items came from - and be sure to return them to the original show. In ballet we do this all the time with props - and if we get around to selling a show, I make sure to remind the PM that if we sell Show X, props a, b and c also go with Show Z, so be careful about what goes in the sale inventory.
Of course this doesn't cover the whole measurements situation, but once you pull your stock, you can always measure before fitting on someone. Hoping you're not the one stuck doing fittings either?