Maximum luggage dimensions when traveling Platzkart?

Usually the luggage in platzkart should be that size it could be hidden under the sleeping place, @EugeneO is right. But there are some details you can use in your way. Let's take a look on the classic railcar from inside:

As you can see, there is additional place above the second level of the bunk beds. There is non-documented agreement, that a person from first level places one's luggage under the bunk bed, and a person from second level places one's luggage above both bunk bed (left upper corner of the photo).

That area could contains a lot of things, usually some touristic stuff easily can be held there. I suggest you to use that place, as it's very handy for unusual sizes, and not so popular among passengers (and if you but tickets for upper place, it's all yours).

The official rules just say a maximum of 180cm as a sum of all dimensions, and 36kg (e.g see here in Russian, unfortunately can't find an official rzd.ru reference with a quick search, but all unofficial pages I'm finding are showing the same numbers).

Obviously, the baggage must fit in the provided space. This thread quotes 120?40?50 dimensions for the compartment (in cm), but take it with a grain of salt.

Also, see here - apparently on many long-distance trains you can also have a "checked luggage" service for an additional fee (never used it myself so can't comment on it further).