Random changes in Icon, winows sizes

I have vision problems, so I like to choose the size of the icons on my desktop. Sometimes, my desktop opens with the size icons I pre-set, but other times they appear smaller. The same problem occurs in windows on the Internet. There does not seem to be a pattern as to when one size of the other is displayed. I end up havng to adjust the size constantly so I can view the content of the window. When the icons are larger, they appear behind the system taskbar at the bottom of the desktop, sometimes not.

Yesterday, I tried to print a file, but found that the print button in the print window was below the visible part of the screen, but I could not change the window size, so I could not print the file.