Increasing font size in TexShop 4.3

I'd say this is a “hidden feature”. In version 4.23 the behavior is the same: when pressing ⌘=, nothing happens unless the first character in the document is selected (with any number of following characters). The same for ⌘- for decreasing the font size.

Similarly, if the focus is on the editing window and you press ⌘K, doing a choice in the font window that appears has no effect, unless the first character is selected.

This font setting is not permanent and will be lost when closing the window.

Changing the source font size permanently is possible if you follow the answer I got by Richard Koch. Here it is:

Go to TeXShop Preferences, and under the Source tab the first item is Document Font. Push the "Set" button. Two things happen. A Small window comes up showing you some sample text, and the font panel comes up. DO NOT CLICK OK in the sample yet.

Using the font panel, select the font and size you like. The sample will change, showing the new results. When you are happy, click OK in the sample. And then, click OK at the bottom of the preference dialog.

(I am using TeXShop in a brand new MacTeX 2020 installation and felt the answers here were not clear, so I decided to post a new answer.)

Open TeXShop. Go to TeXShop--->Preferences--->Source. Under Document Font, click Set.... A box called Sample Text and a box called Fonts appear. You need to delete all the text in the Sample Text box first. Then select the font you want in the Font box. Now type some text in the now empty Sample Text box (empty because you just deleted the text). Click OK in the Sample Text box. Then click OK again to close the menu.