Thanks everyone! Some quick responses...
Never called a "heavy cruiser" in any onscreen TV episode.
This is the confirmation I was fishing for - many thanks for stating it!
I don't think there is one onscreen, except possibly Franz Joseph graphics glimpsed in the OS movies. In "A Taste of Armageddon" Enterprise was called a "star cruiser." In TMOST it was a "space cruiser." I think "heavy cruiser" came from the FJ plans and technical manual.
This is extremely helpful - the scripts give us 'cruiser', just not 'heavy cruiser'. Many thanks!
In the same film, one of the Klingons refers to the Enterprise as a ‘Federation battle cruiser.’
I probably should have been clearer in my original wording - I was looking for a reference
in the TV show, not in the movies. Although even allowing for this, the reference you share isn't necessarily definitive.
For me, the use of the images from Task Force Games or FASA boardgames or role-playing games is not going to make for a definitive canonical point; it's still 'within range' to reject these materials in our fanon. If Kirk says aloud "heavy cruiser", we have a more difficult task rejecting it. If it appears in a visual borrowed from another source, that doesn't (to me) automatically hurdle over the fanon barrier (especially not a Franz Joseph reference, because of the ambiguity of Joseph's reference books and the sheer non-canonicity of anything made by Task Force Games). We have interpretative choices: since we know these game sources from our world, which is strictly incompatible with the fictional world of
Star Trek, the meaning we take from their appearance is ambiguous, and open to fanonical interpretation. It exists in a fascinating grey area.
The Klingon's verbal reference to 'battle cruiser' on the other hand, is clearly canonical. But all it adds to the canon is the Klingon's preferred designation, of course.

I do greatly appreciate the inclusion of the image, though! This is super-helpful. I had never spotted this before. Thanks!
I only remember the Ambassador class Horatio being called a heavy cruiser by Data
Oh, and thanks for this, as I had thought it was Worf you namechecked 'heavy cruiser' in "Conspiracy", but I see you are correct that it is Data. Worf flags the other ships as 'frigates'. That's what kicked this off, as I'm writing the WAM for "Conspiracy", and so I was digging into the rather odd mentioning of ship designations in this episode. I'm pretty sure the term 'frigate' doesn't appear anywhere else in 20th century Trek...
Many, many thanks for all these helpful comments!