First thing I would try is to change the font settings within the driver, usually in the Printing Preferences -> Advanced tab, under Font. If it is set as Resident Font, change it to Download Font, & vice-versa.
Other than that, there is no way of making the font bigger/smaller through the driver. Never seen it done.
Blackcat is right, in that usually proprietary software programs have some sort of 'print setup' menu in there somewhere, where font sizes, types and page layout settings can be adjusted or setup for different printers.
Remember.... the machine just prints what it is told to print. Dont let them put one over you. This should not be your problem!

If the machine is working as it should, leave it alone. Put it back on them.
Another thought...... If it is a proprietary program, see if there is a basic list of printers built into the program. Maybe there is a basic HP Laserjet driver you could try, as it would rip differently. Also, try a different driver for your Sharp, eg PCL6 or 5c etc etc. Just random thoughts. I still stand by what I mentioned above..... not your problem.