You have to take the expand bureaucracy policy, you will then get the chance to focus on expanding the size or ability.

yes, i tried that, but after i clicked that i never heard or saw something about it again, so what does it do and where can i check the progress that it makes?

expand bureaucracy takes along time to fire the event and at the rate it works i have no idea how long it will take, i read the description but i still didnt understand how to up it, ive taken to just have a vassel do the land grabs then i intergrate them at the same time uping my bureaucracy as much as possible, dunno maybe one of the VeF crew could give us a full run down on how it works and how to up it faster.

Expand the bureaucracy triggers ever few years, I'm not sure the exact number but it will trigger. You get to either expand it or increase the quality of it, and I believe either option increases that value by 1 point. I don't believe there is any other way to increase bureaucracy, though I know some unique decisions like the Ottomans making Constantinople their capital increases it.

Another way to deal with bureaucracy is too not increase your capcity, but rather decrease the provinces that you need to manage. If a province has some land controlled by nobles and there is at least light serfdom, those nobles will govern the land for you and it won't contriubte to the provinces that need buercratic management. So try to pick the noble options when you take provinces, my large empires tend to have light serfdom in almost every province as this greatly decreases the amount of provinces your bureaucracy deals with. However I'm not too sure about the bad effects from having a high noble privilege, I haven't seen any directly though I'm sure there must be some drawback to it. I'm thinking it might decrease meritocracy, as this would make a lot of sense. I wish the whole beauracracy, land ownership and serfdom mechanics were explained better, as it seems like a really intruging system.