I Install jitsi meet on centos 7 using docker i want to increase the size of index page div tag

i Want to increase size of That image which is in the header background which is in the background with black colour

You have an SSI include at the end of the “head” section in html, which is for the file “plugin.head.html”.
This is loaded after everything else in head, so any changes there will have priority.
You can create a file .jitsi-meet-cfg/web/custom-plugin.head.html and add the CSS changes in a style tag there. (I never used it, but I do use custom-title and custom-body templates)

P.S.: maybe you need to add a new volume for this in the docker compose:

- ${CONFIG}/web/custom-plugin.head.html:/usr/share/jitsi-meet/plugin.head.html

or otherwise you can try just adding your custom CSS to .jitsi-meet-cfg/web/custom-body.html inside a style tag - it should work OK, it will once again be after all the other styles.