#53 - Bathroom furniture - Although bathroom fittings has been listed already, the store seems to be full of very specialized departments, so I'm listing bathroom "furniture" separate from "fittings' after it was mentioned by Mr. Rumbold.

#54 - Repair department - A department for doing the "repairs" and alterations to clothing. In S2E5, Mr. Humphries says "Don't worry sir, our repair department will patch that up for you before you leave".

#55 - The cleaning department - We often see the cleaning crew on the department floor either early in the morning or late at night. For example, they are there after hours in S2E1 during Mr. Grainger's dinner. They are there early in the morning (at the beginning of the episode) in S2E2 and S4E1.

#56 - The Joke department - In S5E5, Miss Brahms finds out she needs to wear "buck teeth" in order to look like one of the shareholders. Mr. Lucas says, "well we sell those in the joke department".

#57 - The Wig Boutique - This is where Mr. Grainger gets his wig so he can dress up as a shareholder in S5E5. 

Miss Brahms: "Well, we fix him up with something in the wig boutique!"

#58 - Display department - Apparently responsible for setting up any displays in the store, which occurs in S6E1:

Mr. Rumbold: "The royal party are almost here, you ladies from display should have finished by now!"

I believe that buildings in England can only be up to a certain height, so Grace Brothers would have to have been a very wide building.

@basilbjr Good observation, I did not know that was the case.

We do indeed see that there are very likely multiple departments per floor.

For example, we are shown several times that the floor extends far beyond the cameras in the ladies and gents department. We see customers walk off that way a few times, and Mrs. Slocombe runs off behind the cameras while being chased by Mrs. Peacock.

I've no doubt that there are still more departments mentioned in the show that have not been posted here yet. If anyone does a full watch through of the series, feel free to list any more here that you come across.