Example of changing the buttons position: https://www.b4x.com/android/forum/threads/b4xdialog-order-buttons.101686/#post-638451
You can use similar code to change the widths.
Changing the height will be more complicated as you will need to change other things as well. Same is true for the title height. It is possible but you will need to make more changes.
You can see the relevant code in B4XDialog.ShowCustom.