I think what you may be looking for can be done by using dconf-editor. If you do not have it installed, you can do so in Terminal with this:
sudo apt install dconf-editor
Once installed, open dconf-editor and in the left pane navigate to org> gnome> desktop> interface then look for 'text-scaling-factor' in the right pane. The default is, of course, set to 1.0. You can put in 1.25 or 1.50 then hit tab and the change will take effect immediately. You can also search with the editor for other scaling options, too. One word of caution...be very careful using dconf-editor as many a desktop has been botched in one way or the other using the tool.
Hope this helps and Good Luck!!!
EDIT: I just saw Sean's comment to your question and that may be closer to what you're looking for...