See this thread on Resizing Fonts.
I don’t know why the space width is not changing. Maybe InDesign gets the space from some parameter for right to left fonts. All I can think of trying is Format, Settings, Ranges, Average Character Width, Calculate. You may have to change the version to 3 to do this. If you change the space width, you should also change the width of the no break space (decimal codepoint 160)
Looking at MikeHebrew, which is the only Hebrew font I have, I note that he has set the space width to 598 and the no berak space to 265 while the units per em is only 1000. The space width should usually be about 1/4 or 1/5 of the em square, i.e. about 200 in this case Arial uses a value of 569 while Times New Roman uses a value of 512 while the units per em value is 2048. I don’t know if Hebrew fonts require more spacing.