Unfortunately it looks like CKII builds character portraits from parts/layers. So it seems to me you cannot easily get the portrait of just your face without outside tools. You seem to indicate you have tried ingame, but as you say any portrait ingame is going to have the crown on it, etc, and might be small.
I found links on modding portraits that explain how they work, and a tool that could potentially be useful - I dunno. The forum will not allow me to post links of any kind - not even a link to the forum post on modding portraits, or links on the paradox wiki. The forum software thinks it is spam even though it's to paradox stuff. But if you look at paradoxwikis there is an article Portrait_modding which has the links you need as well as some info.