Unfortunately, although there is an "options" button on the OSK, it doesn't seem to have any option for changing the font.
Also, even if I stretch the OSK to fill the screen, the size of the keys gets larger but not the size of the letters on them. The letters are easy enough to see, but Hebrew uses what are called "vowel points" which are tiny dots that go in various positions under, over, and within the letters. It's the dots that are impossible to make out.
I'm using Windows 8.1, by the way.
, clicking
All Programs, clicking Accessories, clicking
Ease of Access, and then clicking On-Screen Keyboard.