Right now, I can easily use a 10 gallon for a hospital tank. I have a sponge filter in it and a hang on (witout carbon). So, it has a ton of filtration to keep it super clean.
I want the smallest tank possible for a hospital tank. That way I have to spend the minimal amount on medication... smallest water changes, smallest amount of salt to add, etc.
I may have to bump it up later when fish get bigger. But, at that time... I hope to not have to use a hospital tank much at all.
Also... I got the 10 gallon tank with a glass top for $10 I think. Air pump for $3. Bought a new sponge and the hang on filter I had around the house.
I used to have 20 gallon for a hospital tank... that turned into a grow out tank. So, I put another 20 gallon under it on the stand. Now thats a grow out tank. So, no more tanks that have any possibility for anything else but a hospital tank.