I needed to combine my elderly cat's medications into a single capsule to mask the taste and reduce handling stress.
Her new pain relief came as a liquid or powder (requiring splitting into 6+ doses) - she froths and stops swallowing at any taste of medication, ruling out liquid or mixing into food/treats.
With the vet's approval to give medications together, I calculated the smallest size 4 empty gelatin capsule should fit the pain relief powder, stress relief powder, and a small tablet, and be small enough for her to swallow. I also bought size 3 as a backup for larger doses.
I drilled holes in a small wooden block to hold capsules upright, used a tiny funnel for the powders, then added the tablet (sometimes broken up) into the cap before closing. Wrapped in a thin layer of Pill Pockets, it was easier to administer than any other method. For lower doses, I trimmed the open capsule with scissors - it no longer locked but the Pill Pocket coating kept it together, resulting in a shorter capsule.
Size 4 has been ideal, though I'm glad I have size 3 on hand for higher doses.