My 27-inch iMac made things too small for my old eyes and I changed the screen resolution from the default of 2560x1440 to 1920x1080. The menubar is now 0.30" wide and easy to read, as are most text in programs. I still have plenty of screen real estate and the change in res did not affect the sharpness of text or images.
There is an option in System Preferences >Displays to show a screen res icon in the menubar that looks like this in use:
If you use that option you can quickly change res later if something needs a larger res.
Thanks Allan for your help. I know the change in resolution works but was hoping to keep the full screen and still enlarge the tool bar. I have been into an apple shop today and they have shown me a zoom function in settings/accessibility which you can use by pressing option & command & plus to make it bigger and the same buttons with minus to put back to original size. Between the 2 options we will make it work. I appreciate your reply. Julie