Hi Uncgirl,
The CTRL + works for me to increase the font size even in the “post reply” field, so I would say it’s possible. The following is a tutorial I found on the internet for increasing font size using a Safari Browser. Let me know if this helps.
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The size of the text displayed on web pages within your Safari browser may be too small for you to clearly read. On the flip side of that coin, you may find that it is too large for your taste. Safari gives you the ability to easily increase or decrease the font size of all text within a page.
First, open your Safari browser.

Click on View in your Safari menu, located at the top of your screen. When the drop-down menu appears, click on the option labeled Make Text Bigger.
You will notice that all of the text within the web page you are viewing has gotten larger. To increase the size again, just repeat these steps.
Please note that you can use the following shortcut keys in lieu of clicking the menu choice: Apple and + (Plus).

Click on View in your Safari menu, located at the top of your screen. When the drop-down menu appears, click on the option labeled Make Text Smaller.
You will notice that all of the text within the web page you are viewing has gotten smaller. To decrease the size again, just repeat these steps.
Please note that you can use the following shortcut keys in lieu of clicking the menu choice: Apple and - (Minus).

The Safari browser contains two buttons which can be used to increase or decrease text size. These buttons are placed on your main toolbar, in between the Back/Forward navigation buttons and the Refresh button. They do not appear by default. You must modify your browser settings in order to make these buttons available.
To do this, click on View in your Safari menu, located at the top of your screen. When the drop-down menu appears, click on the option labeled Text Size.

You will now see two new buttons displayed on your Safari toolbar, one labeled with a small "A" and another with a bigger "A". The small "A" button, when pressed, will decrease the text size while the other button will increase it. When using these, the same behavior will occur as when you use the options detailed in Steps 2 and 3 of this tutorial.