Is there a shorter version of the text I can use to make it fit?

Hmm... let me think...
BTW, "51 стаков" and "2 стаков" is incorrect by grammar. Correct form is "51 стак" and "2 стака". NWM, russian grammar can drive you crazy.
Oh, "[number of stacks] стаков из [stack size]" incorrect too. I mean "из" does not fit the meaning.
Briefly speaking, most correct by grammar and meaning (and simplest in coding i believe) and shortest version is "по".
Example: "55 по 5", "2 по 5", "1 по 1" etc.

Thanks a lot. I've just changed the translations to just that, and you should be able to use it in the next alpha release, 9.1.6-24-gc237f0e-bcc
Auctionator-9.1.6-24-gc237f0e-bcc.zip