You can do this with an expression for the size in the labelling dialog.
First click the expression button at the far right of the size setting - here its yellow because its active (at first the button will look like the others):
Then set it to something like this, depending on what size you want and the name of the attribute of your towns - here I want London labelled at size 24 and everything else at size 9:
This will get complicated if you have a lot of towns with different size labels. Create a new attribute column in your layer called "LargeLabel" or something, and set to 1 for those you want labelled large, and adjust the expression accordingly.
If you want to label towns with large populations larger, and you have "population" in your layer attributes, you can do something like
if(population>100000, 24, 9) to label them with larger text.
If you want to scale label sizes according to the value in the dialog, you can use
@value in an expression. This expression will set London in text which is 3 times the size of the "9.000" in the dialog box, and other labels in size 9.000 - and if I change the "9.000" everything scales up or down proportionally:
@value * if(name='London',3,1)