Could be in Yast2-bootloader-kernel parameters-console resolution set the appropriate resolution
For font size and style adjustment if you are using KDE use QT5 Settings you may use it probably as root.
I just take a trying according to your method. I found that the resolution I selected for graphic console only affect the screen of booting process. At the end of booting when login form present, all my selections in yast2-bootloader do not change the looking of the login form.
All things are ok if I use nouveau driver. I get problem when I use NVIDIA driver.