the extent and width should be calculated values.

That would change the example code to something like

LoginModel class>>defaultSpec
<spec: #default>
^SpecLayout composed
      newColumn: [ :col | 
        col 
          newRow: [ :row | 
            row 
              add: #usernameLabel width: self labelWidth; 
              add: #usernameField ] 
          height: self inputTextHeight; 
          newRow: [ :row | 
            row 
              add: #passwordLabel width: self labelWidth; 
              add: #passwordField ] 
          height: self inputTextHeight ]; 
       yourself 

LoginModel class>>labelWidth
^((StandardFonts defaultFont widthOfString: 'Password') max: 
(StandardFonts defaultFont widthOfString: 'Username'))+12

Please note that there are still several places in Spec where 

hardcoded values like 12 are used instead of semantic values.

This value should actually be dependent on font size.

Having the label texts twice in the code should also be avoided.

LoginModel class>>textFieldWidth
^(StandardFonts defaultFont widthOfString: 'longest.email@addressThatShould.fit')+12

LoginModel>>initialExtent
^(3*12+self class labelWidth+ self class textFieldWidth) @ (5*self class buttonHeight)

There are several calculated values missing:

width = outside border + labelWidth + label field separator+fieldWidth+outside border

height = dialog title + textInput height + separator + textInput height+ separator+button height + outside border

Stephan