The size of the text in a MessageBox is determined by the system (at least by default). You can change it, but not very easily.
https://www.codeproject.com/Articles/601900/FlexibleMessageBox-A-flexible-replacement-for-the
https://web.archive.org/web/20091208060754/http://msdn.microsoft.com/en-gb/magazine/cc188920.aspx
Or, you can create your own form with the appropriate buttons and text boxes, etc.