Shabnam Watson shows how to change the font size for a couple tiny parts of SSAS:
The small font size may be difficult to read for some people. The easiest and fastest way to change the font size is to click somewhere in the formula bar and use your mouse middle wheel while holding down the Control button on your keyboard. However, the font size will go back to the default size next time you open SSDT.
If you wish to change the font size permanently, you can use the Tools menu. Click on Tools –> Options –> Fonts and Colors –> Environment and choose “Business Intelligence Designers” from the drop down menu. Here you can change the font size. Your changes will not take effect until you restart SSDT.
Unfortunately, the surrounding chrome doesn’t change size, so there are hard limits of how far you can scale up.