You can change the fonts used on the login screen by setting one of the following font resources in Xresources:
Dtlogin*greeting*fontList—font for welcome messages
Dtlogin*labelFont—font for push buttons and labels
Dtlogin*textFont—font for help and error messages
To list the available fonts, type:
xlsfonts [-options] [-fn pattern]
The following example uses a large font for the welcome message (the value you specify must be contained on one line):
Dtlogin*greeting*fontList: -dt-interface \ system-medium-r-normal-xxl*-*-*-*-*-*-*-*-*: