There are some options to increase font size, depending on the Grade and Measure you choose, and the type of computer platform (Windows/Mac).
CCSS Reading and Reading Comprehension measures: When a student logs in to take their selected reading assessment, and the measure appears on the screen, look in the upper right corner. Here you will see three As of varying sizes. Select a larger size A, and the print on the assessment will increase.
Math (NCTM) and CCSS Math measures: For these, after the student logs in, I would use your computer’s zoom capability to enlarge the size of the math problems to what a student prefers/needs. For a Mac, you can hold down the Command key and press either the plus or minus key to zoom in/out, respectively (i.e., Cmd +/-). There are also read-aloud options for online versions of these tests. If printing them out, I would make copies at a larger size (label with measure name, grade, test form number, save them, and possibly laminate if that is a resource available).