size of the letter

The System Preferences > Display settings can be used to scale the whole screen larger; larger text, larger images, and such.

There are other options for viewing your Mac available, too.

Vision Accessibility - Mac - Apple

The trackpad can also be used to zoom, as well. (I use that heavily.)

And the command+ and command- mentioned earlier, too.

Note sure if I fully understand.

But pressing Command + to make larger and Command - to make smaller might work