The main variants are 44 and 47mm.
The smaller varieties aren't very popular on forums.
I have never been able to fathom their numbering system.
There are a few case variants.
Luminor - crown guard.
Luminor 1950 - more historic with engraved crown guard.
Radiomir - no crown guard, often have wire lugs.
Radiomir 1940 - latest case shape. Sort of a hybrid between the 50s case and radiomir case, no wire lugs.
Enthusiasts tend to go for base models, handwound without complications and usually a sandwich dial because they find them more historically in keeping with the Panerai of old.
Really though, it's a case of finding what you like the look of in other threads and reading up as there's such a lot of small to huge variations.