How do I fix the text size/positioning issue in Powerpoint?

You have left out a few details. What OSX version are you using? What program are you using on OSX? Keynote? LibreOffice? MS Office? Which version?

The short answer is you will never be able to avoid opening a MS Office file with some loss of fidelity on a Mac if it was created on Windows, even if you shell out for an Office365 subscription. I don't know whose fault this is but I strongly suspect MS is doing things to their file formats to intentionally break the ability of third party apps to access data that was created using MS software. This is a blatant bullying tactic on MS's part and it sells a lot of copies of Office.

There are a few workarounds. You can run the exact same Windows version of Office on your Mac using either Crossover (wine emulation), Parallels or Bootcamp. Any of these options might lead to you spending money buying stuff from MS. Another option is to live with the small loss of fidelity and just keep using the program you like best. Every now and then you will have to stretch some text.

I went through this with my wife some years back. My daughter's teachers were sending out assignments in some MS format that came up either blank or as fuzzy lines when my wife tried to view it. I installed Office 2011 on her Mac purchased for $10 through my company "home use" program. It was the biggest waste of $10 I have ever witnessed in my adult life. The software installed what had to be a DOZEN resource-sucking icons in her dock. It deleted fonts that were needed by LibreOffice and I wound up reinstalling LibreOffice to get it working again after the MS Office install. I had to go through activation to prove I wasn't some kind of thief. Installing MS Office is one of the least satisfying things I've ever had to do on any Mac.

If you decide you really must use MS Office, I suggest the $79 for four years Office365 student subscription. It allows you to install a copy on your Mac and if you later decide you need to run the windows version inside parallels, your same subscription would still be valid for the Windows version. Just remember, buying MS Office because of a prof who sends out stuff in MS format is applying a 4 year solution to a 1 semester problem.