Do not be concerned on thos numbers they are more for manufacturers benefit not ours.
What is important is the size of your case,or at least the space you have for the graphics card. It might be a good idea to measure the length of your current graphics card and any space at the back of the card.I see you have posted the cards length (21cm's) so what space do you have behind it.
That way we will have an idea of overall length. I suspect even though you have a small case you should have know problem installing a GPU of your choice.
Very important is what is your budget.