I don't know what the solution would be unless you could get some kind of OS detector script and have it choose the fonts size based on that. The other option is to use images with the text part of the image then it would be the same on both, the problem being that this is can be difficult for those who need larger fonts for visual impairments.
Mac screen resolution is based on the print media system and is 72dpi resolution, dots per inch, but Windows is based on computer [graphical I think, ppi pixels per inch] and is usually 96dpi. The same fonts will then be a different size, on the mac 1/6th of 72, on the PC 1/6th of 96. So, the windows font ends up bigger.
I think I got all that right
Some have said they see the same size on some sites in Mac and PC but no one has put a link to the page so I can see what they did to get that.
Ruth