div to be smaller = > shall I make width =some_percent % to make smaller ?
div to be in the **middle ** = > what to do with this ?
message to be at the center of the div = > what to do with this ?
So far, you haven’t mentioned the version of bootstarp that you are using. Sorry, but I’m not into “guessing” my time away.
Don’t change the original Bootstrap code. If modifications are necessary, write code on a local stylesheet that overrides the bootstrap code. Do you know how to do that?
Display your site or test page in your browser. Right-click the page and “show source”. Copy the code that is shown to a file. Open the copied page in another browser tab to be sure it matches the original page. Edit that copied file while displaying it in another browser window. Gradually eliminate unnecessary HTML until you have a minimal working page that demonstrates the problem. (That may take a couple of hours). Post the minimal page along with the appropriate supporting bootstrap code in a message including URLs to needed resources. When you can open the page in your browser and see the problem, the post the remaining code in a message.
If all else fails, take a class in HTML and CSS and learn to use your browser’s developer’s tools so you can learn to read bootscrap’s CSS.
Offhand, it sounds like a job for a table/table-cell or flex, depending on the context.