There are multiple places in MacPractice where you can adjust font sizes.
Schedule
Use the Appointment Display settings to adjust font sizes and colors on appointments. 
Sidebar
The sidebar can also be adjusted by going to the Preferences. 
EMR/EDR
Forms
You can change the font style and size for any non-codified element in EMR or EDR. There are default settings in the Preferences and you may also edit this per Form Section. This will display on the Form as you fill it out.
You can also use the Form Palette to zoom in when working with a patient form.
Narratives
The Narrative font can also be changed. There are default settings in the Preferences and you may also edit this per Form Section.
EHR
The template or form side of an EHR form cannot be changed. However, you may contact MacPractice support team for assistance with changing font sizes for the Narrative.
Reports
Many reports have the option to change font size while viewing results. Use the font button on the lower left of your reports window. 
Ledger, Clinical Tab, or Other MacPractice Windows
Unfortunately, some windows in MacPractice do not allow for adjustable font sizes. You might consider adjusting your screen resolution in System Preferences. Click on the Apple icon to select System Preferences > Displays. Change your display settings to Scaled and test out the setting that works best for you. 
Apple also has a Zoom feature which you can use if you only occasionally need to zoom in on specific font. This is located in System Preferences, in the Accessibility settings. 