For squeekboard, you could try a different, wider layout (even create it yourself); it will still shrink, but maybe the display will be more usable for you. See the links at: Use only Terminal Keyboard?
For the lockscreen display, I don’t know how to change the size independently of the general resolution setting. If it’s possible, you’ll probably have to ping one of the Purism staff/devs. They may have a different solution for the keyboard, as well.