There are several ways to set the size or magnification for text in different parts of the mac experience. Here are some of the basics, and in the order I think it's best you try:
Screen resolution in System Settings.
Just click on the little size icons and the display will immediately change the number of pixels, so; fewer pixels means bigger icons, windows, menu bar, everything. Set it to what works for you. You can also have it show more resolution option by ctrl-clicking (IIRC), and it will show resoution numbers. (Like, I have my displays set to 1600x900, lets me read text on my 55" from across the room.)
Finder windows have options for setting sizes of icons, font (unfortunately limited to 16points) etc for the currently active window. Cmnd-J or 'View' menu. It also includes an option to 'use as default'. This also works on how the desktop items are shown.
Individual apps might have their own settings for display size. Especially relevant for browser apps. In Firefox, I can set a zoom factor for graphical items and another zoom factor for all text on a page, and also set a preferred default. I have all my pages open in 130% zoom, makes most web sites fill the screen. Browsers have plus or minus buttons or usually react to cmnd-plus or cmnd-minus.
IHTH