I am a tank and I use your addon in dungeons, but when all monsters are casting, their castbar blocks the healthbars vision.
Can you make more customization for the castbar, for example, reduce the size.
Thank you.

This can technically already be done through the Theme Customization. Although I do understand that because the castbar is comprised of several different elements I can be a bit bothersome to do so. And at some point this will hopefully be condensed into a single, or at least less, elements that have to be edited and moved around. But I can't really give a timeframe for when that will happen.

I tried, but could not make the border and bar the same size.
I found only 3 lines in settings for the castbar, maybe there is more.

In some cases, due to how some textures are set up, you would have to also adjust the Offset Y position of them. The castbar border is one of these cases. The Theme Customization system is essentially still in alpha and hasn't had much improvements made to it, in part due to issues like this with textures, but also because there are slight differences between themes in general. Which makes it harder to do as an overall system without changing somethings about how themes are currently set up.

If you don't have any other Theme Customizations right now here's an import string with a smaller castbar height set as an example, just so you can see what I mean.

^1^T^SNameOnly^T^t^SDefault^T^Scastnostop^T^Sy^T^Svalue^N11^Stype^Soffset^t^Sh^T^Svalue^N-28^Stype^Soffset^t^Senabled^T^Svalue^B^Stype^Sactual^t^Sx^T^Svalue^N0^Stype^Soffset^t^Sheight^T^Svalue^N-28^Stype^Soffset^t^Sanchor^T^Svalue^SCENTER^Stype^Sactual^t^Sw^T^Svalue^N0^Stype^Soffset^t^Swidth^T^Svalue^N0^Stype^Soffset^t^t^Scastborder^T^Sy^T^Svalue^N11^Stype^Soffset^t^Sh^T^Svalue^N-28^Stype^Soffset^t^Senabled^T^Svalue^B^Stype^Sactual^t^Sx^T^Svalue^N0^Stype^Soffset^t^Sheight^T^Svalue^N-28^Stype^Soffset^t^Sanchor^T^Svalue^SCENTER^Stype^Sactual^t^Sw^T^Svalue^N0^Stype^Soffset^t^Swidth^T^Svalue^N0^Stype^Soffset^t^t^Scastbar^T^Sy^T^Svalue^N4^Stype^Soffset^t^Sh^T^Svalue^N-6^Stype^Soffset^t^Senabled^T^Svalue^B^Stype^Sactual^t^Sx^T^Svalue^N0^Stype^Soffset^t^Sheight^T^Svalue^N-6^Stype^Soffset^t^Sanchor^T^Svalue^SCENTER^Stype^Sactual^t^Sw^T^Svalue^N0^Stype^Soffset^t^Swidth^T^Svalue^N0^Stype^Soffset^t^t^t^SWidgetConfig^T^t^t^^

Doing a bit of cleaning up and closing issues that are stale, or has been resolved by never confirmed to be resolved. If you believe this is in error, please comment and I'll re-open the issue again.